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<?xml version="1.0" encoding="UTF-8"?>
<chapter xml:id="mvc"
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<title>Web MVC framework</title>
<section xml:id="mvc-introduction">
<title>Introduction to Spring Web MVC framework</title>
<para>The Spring Web model-view-controller (MVC) framework is designed
around a <classname>DispatcherServlet</classname> that dispatches requests
to handlers, with configurable handler mappings, view resolution, locale
and theme resolution as well as support for uploading files. The default
handler is based on the <interfacename>@Controller</interfacename> and
<interfacename>@RequestMapping</interfacename> annotations, offering a
wide range of flexible handling methods. With the introduction of Spring
3.0, the <interfacename>@Controller</interfacename> mechanism also allows
you to create RESTful Web sites and applications, through the
<interfacename>@PathVariable</interfacename> annotation and other
features.</para>
<sidebar xml:id="mvc-open-for-extension">
<title><quote>Open for extension...</quote></title>
<para>A key design principle in Spring Web MVC and in Spring in general
is the <quote><emphasis>Open for extension, closed for
modification</emphasis></quote> principle.</para>
<para>Some methods in the core classes of Spring Web MVC are marked
<literal>final</literal>. As a developer you cannot override these
methods to supply your own behavior. This has not been done arbitrarily,
but specifically with this principle in mind.</para>
<para>For an explanation of this principle, refer to <emphasis>Expert
Spring Web MVC and Web Flow</emphasis> by Seth Ladd and others;
specifically see the section "A Look At Design," on page 117 of the
first edition. Alternatively, see</para>
<orderedlist>
<listitem>
<para><link
xl:href="http://www.objectmentor.com/resources/articles/ocp.pdf">Bob
Martin, The Open-Closed Principle (PDF)</link></para>
</listitem>
</orderedlist>
<para>You cannot add advice to final methods when you use Spring MVC.
For example, you cannot add advice to the
<literal>AbstractController.setSynchronizeOnSession()</literal> method.
Refer to <xref linkend="aop-understanding-aop-proxies" /> for more
information on AOP proxies and why you cannot add advice to final
methods.</para>
</sidebar>
<para>In Spring Web MVC you can use any object as a command or
form-backing object; you do not need to implement a framework-specific
interface or base class. Spring's data binding is highly flexible: for
example, it treats type mismatches as validation errors that can be
evaluated by the application, not as system errors. Thus you need not
duplicate your business objects' properties as simple, untyped strings in
your form objects simply to handle invalid submissions, or to convert the
Strings properly. Instead, it is often preferable to bind directly to your
business objects.</para>
<para>Spring's view resolution is extremely flexible. A
<interfacename>Controller</interfacename> is typically responsible for
preparing a model <classname>Map</classname> with data and selecting a
view name but it can also write directly to the response stream and
complete the request. View name resolution is highly configurable through
file extension or Accept header content type negotiation, through bean
names, a properties file, or even a custom
<interfacename>ViewResolver</interfacename> implementation. The model (the
M in MVC) is a <interfacename>Map</interfacename> interface, which allows
for the complete abstraction of the view technology. You can integrate
directly with template based rendering technologies such as JSP, Velocity
and Freemarker, or directly generate XML, JSON, Atom, and many other types
of content. The model <interfacename>Map</interfacename> is simply
transformed into an appropriate format, such as JSP request attributes, a
Velocity template model.</para>
<section xml:id="mvc-features">
<title>Features of Spring Web MVC<!--I moved Features of Spring Web MVC before Pluggability of other MVC implementations. You want to highlight your own imp. first.--></title>
<!--Second line of sidebar refers to JSF; don't you mean JSP? Other refs in this context are to JSP. Also note, sidebar is read-only.-->
<xi:include href="swf-sidebar.xml"
xmlns:xi="http://www.w3.org/2001/XInclude" />
<para>Spring's web module includes many unique web support
features:</para>
<itemizedlist>
<listitem>
<para><emphasis>Clear separation of roles</emphasis>. Each role —
controller, validator, command object, form object, model object,
<classname>DispatcherServlet</classname>, handler mapping, view
resolver, and so on — can be fulfilled by a specialized
object.</para>
</listitem>
<listitem>
<para><emphasis>Powerful and straightforward configuration of both
framework and application classes as JavaBeans</emphasis>. This
configuration capability includes easy referencing across contexts,
such as from web controllers to business objects and
validators.</para>
</listitem>
<listitem>
<para><emphasis>Adaptability, non-intrusiveness, and
flexibility.</emphasis> Define any controller method signature you
need, possibly using one of the parameter annotations (such as
@RequestParam, @RequestHeader, @PathVariable, and more) for a given
scenario.</para>
</listitem>
<listitem>
<para><emphasis>Reusable business code</emphasis>,<emphasis> no need
for duplication</emphasis>. Use existing business objects as command
or form objects instead of mirroring them to extend a particular
framework base class.</para>
</listitem>
<listitem>
<para><emphasis>Customizable binding and validation</emphasis>. Type
mismatches as application-level validation errors that keep the
offending value, localized date and number binding, and so on
instead of String-only form objects with manual parsing and
conversion to business objects.</para>
</listitem>
<listitem>
<para><emphasis>Customizable handler mapping and view
resolution</emphasis>. Handler mapping and view resolution
strategies range from simple URL-based configuration, to
sophisticated, purpose-built resolution strategies. Spring is more
flexible than web MVC frameworks that mandate a particular
technique.</para>
</listitem>
<listitem>
<para><emphasis>Flexible model transfer</emphasis>. Model transfer
with a name/value <interfacename>Map</interfacename> supports easy
integration with any view technology.</para>
</listitem>
<listitem>
<para><emphasis>Customizable locale and theme resolution, support
for JSPs with or without Spring tag library, support for JSTL,
support for Velocity without the need for extra bridges, and so
on.</emphasis></para>
</listitem>
<listitem>
<para><emphasis>A simple yet powerful JSP tag library known as the
Spring tag library that provides support for features such as data
binding and themes</emphasis>. The custom tags allow for maximum
flexibility in terms of markup code. For information on the tag
library descriptor, see the appendix entitled <xref
linkend="spring.tld" /></para>
</listitem>
<listitem>
<para><emphasis>A JSP form tag library, introduced in Spring 2.0,
that makes writing forms in JSP pages much easier.</emphasis> For
information on the tag library descriptor, see the appendix entitled
<xref linkend="spring-form.tld" /></para>
</listitem>
<listitem>
<para><emphasis>Beans whose lifecycle is scoped to the current HTTP
request or HTTP <interfacename>Session</interfacename>.</emphasis>
This is not a specific feature of Spring MVC itself, but rather of
the <interfacename>WebApplicationContext</interfacename>
container(s) that Spring MVC uses. These bean scopes are described
in <xref linkend="beans-factory-scopes-other" /></para>
</listitem>
</itemizedlist>
</section>
<section xml:id="mvc-introduction-pluggability">
<title>Pluggability of other MVC implementations</title>
<para>Non-Spring MVC implementations are preferable for some projects.
Many teams expect to leverage their existing investment in skills and
tools. A large body of knowledge and experience exist for the Struts
framework. If you can abide Struts' architectural flaws, it can be a
viable choice for the web layer; the same applies to WebWork and other
web MVC frameworks.</para>
<para>If you do not want to use Spring's web MVC, but intend to leverage
other solutions that Spring offers, you can integrate the web MVC
framework of your choice with Spring easily. Simply start up a Spring
root application context through its
<classname>ContextLoaderListener</classname>, and access it through
its<!--Identify *its*. do you mean root application context's?-->
<interfacename>ServletContext</interfacename> attribute (or Spring's
respective helper method) from within a Struts or WebWork action. No
"plug-ins" are involved, so no dedicated integration is necessary. From
the web layer's point of view, you simply use Spring as a library, with
the root application context instance as the entry point.</para>
<para>Your registered beans and Spring's services can be at your
fingertips even without Spring's Web MVC. Spring does not compete with
Struts or WebWork in this scenario. It simply addresses the many areas
that the pure web MVC frameworks do not, from bean configuration to data
access and transaction handling. So you can enrich your application with
a Spring middle tier and/or data access tier, even if you just want to
use, for example, the transaction abstraction with JDBC or
Hibernate.</para>
</section>
</section>
<section xml:id="mvc-servlet">
<title>The <classname>DispatcherServlet</classname></title>
<para>Spring's web MVC framework is, like many other web MVC frameworks,
request-driven, designed around a central Servlet that dispatches requests
to controllers and offers other functionality that facilitates the
development of web applications. Spring's
<classname>DispatcherServlet</classname> however, does more than just
that. It is completely integrated with the Spring IoC container and as
such allows you to use every other feature that Spring has.</para>
<para>The request processing workflow of the Spring Web MVC
<classname>DispatcherServlet</classname> is illustrated in the following
diagram. The pattern-savvy reader will recognize that the
<classname>DispatcherServlet</classname> is an expression of the
<quote>Front Controller</quote> design pattern (this is a pattern that
Spring Web MVC shares with many other leading web frameworks).</para>
<para><mediaobject>
<imageobject>
<imagedata align="center" fileref="images/mvc.png" format="PNG" width="400"/>
</imageobject>
<caption><para>The request processing workflow in Spring Web MVC (high
level)</para></caption>
</mediaobject></para>
<para>The <classname>DispatcherServlet</classname> is an actual
<interfacename>Servlet</interfacename> (it inherits from the
<classname>HttpServlet</classname> base class), and as such is declared in
the <literal>web.xml</literal> of your web application. You need to map
requests that you want the <classname>DispatcherServlet</classname> to
handle, by using a URL mapping in the same <literal>web.xml</literal>
file. This is standard Java EE Servlet configuration; the following example
shows such a <classname>DispatcherServlet</classname> declaration and
mapping:</para>
<programlisting language="xml">&lt;web-app&gt;
&lt;servlet&gt;
&lt;servlet-name&gt;example&lt;/servlet-name&gt;
&lt;servlet-class&gt;org.springframework.web.servlet.DispatcherServlet&lt;/servlet-class&gt;
&lt;load-on-startup&gt;1&lt;/load-on-startup&gt;
&lt;/servlet&gt;
&lt;servlet-mapping&gt;
&lt;servlet-name&gt;example&lt;/servlet-name&gt;
&lt;url-pattern&gt;/example/*&lt;/url-pattern&gt;
&lt;/servlet-mapping&gt;
&lt;/web-app&gt;</programlisting>
<para>In the preceding example, all requests starting with
<literal>/example</literal> will be handled by the
<classname>DispatcherServlet</classname> instance named
<literal>example</literal>.
In a Servlet 3.0+ environment, you also have the
option of configuring the Servlet container programmatically. Below is the code
based equivalent of the above <filename>web.xml</filename> example:</para>
<programlisting language="java">public class MyWebApplicationInitializer implements WebApplicationInitializer {
@Override
public void onStartup(ServletContext container) {
ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet());
registration.setLoadOnStartup(1);
registration.addMapping("/example/*");
}
}</programlisting>
<para><interfacename>WebApplicationInitializer</interfacename> is an interface
provided by Spring MVC that ensures your code-based configuration is detected and
automatically used to initialize any Servlet 3 container. An abstract base class
implementation of this interace named
<classname>AbstractDispatcherServletInitializer</classname> makes it even easier
to register the <classname>DispatcherServlet</classname> by simply specifying
its servlet mapping. See
<link linkend="mvc-container-config">Code-based Servlet container initialization</link>
for more details.</para>
<para>The above is only the first step in setting up
Spring Web MVC. <!--The discussion below is a little vague about what you're doing, when you do it, and what you're accomplishing. --><!-- Is the next step shown in the next example screen?-->You
now need to configure the various beans used by the Spring Web MVC
framework (over and above the <classname>DispatcherServlet</classname>
itself).<!--See previous sentence. Add info to indicate where you find info that tells you how to configure beans for MVC framework. --><!--Next paragraph, so what are you telling them to *do* here? --></para>
<para>As detailed in <xref linkend="context-introduction" />,
<interfacename>ApplicationContext</interfacename> instances in Spring can
be scoped. In the Web MVC framework, each
<classname>DispatcherServlet</classname> has its own
<interfacename>WebApplicationContext</interfacename>, which inherits all
the beans already defined in the root
<interfacename>WebApplicationContext</interfacename>. These inherited
beans can be overridden in the servlet-specific scope, and you can define
new scope-specific beans local to a given Servlet instance.</para>
<para><mediaobject>
<imageobject>
<imagedata align="center" fileref="images/mvc-contexts.gif" format="GIF" width="400" />
</imageobject>
<caption>Context hierarchy in Spring Web MVC</caption>
</mediaobject></para>
<para>Upon initialization of a <classname>DispatcherServlet</classname>,
Spring MVC looks for a file named <emphasis>
<literal>[servlet-name]-servlet.xml</literal></emphasis> in the
<literal>WEB-INF</literal> directory of your web application and creates
the beans defined there, overriding the definitions of any beans defined
with the same name in the global scope.</para>
<para>Consider the following <classname>DispatcherServlet</classname>
Servlet configuration (in the <literal>web.xml</literal> file):</para>
<programlisting language="xml">&lt;web-app&gt;
&lt;servlet&gt;
&lt;servlet-name&gt;<emphasis role="bold">golfing</emphasis>&lt;/servlet-name&gt;
&lt;servlet-class&gt;org.springframework.web.servlet.DispatcherServlet&lt;/servlet-class&gt;
&lt;load-on-startup&gt;1&lt;/load-on-startup&gt;
&lt;/servlet&gt;
&lt;servlet-mapping&gt;
&lt;servlet-name&gt;<emphasis role="bold">golfing</emphasis>&lt;/servlet-name&gt;
&lt;url-pattern&gt;/golfing/*&lt;/url-pattern&gt;
&lt;/servlet-mapping&gt;
&lt;/web-app&gt;</programlisting>
<para>With the above Servlet configuration in place, <!--Is this something you need to do (in above example)? -->you
will need to have a file called <literal>/WEB-INF/</literal><emphasis
role="bold">golfing</emphasis><literal>-servlet.xml</literal> in your
application; this file will contain all of your Spring Web MVC-specific
components (beans). You can change the exact location of this
configuration file through a Servlet initialization parameter (see below
for details).</para>
<!--See *where* for details? Give x-ref to section talks about how to change the location of the file through servlet init. param.-->
<para>The <interfacename>WebApplicationContext</interfacename> is an
extension of the plain <interfacename>ApplicationContext</interfacename>
that has some extra features necessary for web applications. It differs
from a normal <interfacename>ApplicationContext</interfacename> in that it
is capable of resolving themes (see <xref linkend="mvc-themeresolver" />),
and that it knows which Servlet it is associated with (by having a link to
the <interfacename>ServletContext</interfacename>). The
<interfacename>WebApplicationContext</interfacename> is bound in the
<interfacename>ServletContext</interfacename>, and by using static methods
on the <classname>RequestContextUtils</classname> class you can always
look up the <interfacename>WebApplicationContext</interfacename> if you
need access to it.</para>
<section xml:id="mvc-servlet-special-bean-types">
<title>Special Bean Types In the <interfacename>WebApplicationContext</interfacename></title>
<para>The Spring <classname>DispatcherServlet</classname> uses special
beans to process requests and render the appropriate views. These beans
are part of Spring MVC. You can choose which special beans to use
by simply configuring one or more of them in the
<interfacename>WebApplicationContext</interfacename>.
However, you don't need to do that initially since Spring MVC
maintains a list of default beans to use if you don't configure any.
More on that in the next section. First see the table below
listing the special bean types the
<classname>DispatcherServlet</classname> relies on.</para>
<table xml:id="mvc-webappctx-special-beans-tbl">
<title>Special bean types in the
<interfacename>WebApplicationContext</interfacename></title>
<tgroup cols="2">
<colspec colname="c1" colwidth="1*" />
<colspec colname="c2" colwidth="4*" />
<thead>
<row>
<entry>Bean type</entry>
<entry>Explanation</entry>
</row>
</thead>
<tbody>
<row>
<entry><link linkend="mvc-handlermapping">HandlerMapping</link></entry>
<entry>Maps incoming requests to handlers and a list of
pre- and post-processors (handler interceptors) based on some
criteria the details of which vary by <interfacename>HandlerMapping</interfacename>
implementation. The most popular implementation supports
annotated controllers but other implementations exists as well.</entry>
</row>
<row>
<entry>HandlerAdapter</entry>
<entry>Helps the <interfacename>DispatcherServlet</interfacename> to
invoke a handler mapped to a request regardless of the handler
is actually invoked. For example, invoking an annotated controller
requires resolving various annotations. Thus the main purpose
of a <interfacename>HandlerAdapter</interfacename> is to shield the
<classname>DispatcherServlet</classname> from such details.</entry>
</row>
<row>
<entry><link linkend="mvc-exceptionhandlers">HandlerExceptionResolver</link></entry>
<entry>Maps exceptions to views also allowing for more
complex exception handling code.</entry>
</row>
<row>
<entry><link linkend="mvc-viewresolver">ViewResolver</link></entry>
<entry>Resolves logical String-based view names to actual
<interfacename>View</interfacename> types.</entry>
</row>
<row>
<entry><link linkend="mvc-localeresolver">LocaleResolver</link></entry>
<entry>Resolves the locale a client is using,
in order to be able to offer internationalized views</entry>
</row>
<row>
<entry><link linkend="mvc-themeresolver">ThemeResolver</link></entry>
<entry>Resolves themes your web application can use, for
example, to offer personalized layouts</entry>
</row>
<row>
<entry><link linkend="mvc-multipart">MultipartResolver</link></entry>
<entry>Parses multi-part requests for example to support processing
file uploads from HTML forms.</entry>
</row>
<row>
<entry><link linkend="mvc-flash-attributes">FlashMapManager</link></entry>
<entry>Stores and retrieves the "input" and the "output"
<classname>FlashMap</classname> that can be used to pass attributes
from one request to another, usually across a redirect.</entry>
</row>
</tbody>
</tgroup>
</table>
</section>
<section xml:id="mvc-servlet-config">
<title>Default DispatcherServlet Configuration</title>
<para>As mentioned in the previous section for each special bean
the <classname>DispatcherServlet</classname> maintains a list
of implementations to use by default. This information is
kept in the file <filename>DispatcherServlet.properties</filename>
in the package <classname>org.springframework.web.servlet</classname>.
</para>
<para>All special beans have some reasonable defaults of
their own. Sooner or later though you'll need to customize
one or more of the properties these beans provide.
For example it's quite common to configure
an <classname>InternalResourceViewResolver</classname>
settings its <literal>prefix</literal> property to
the parent location of view files.</para>
<para>Regardless of the details, the important concept
to understand here is that once
you configure a special bean such as an
<classname>InternalResourceViewResolver</classname>
in your <classname>WebApplicationContext</classname>, you
effectively override the list of default implementations
that would have been used otherwise for that special bean
type. For example if you configure an
<classname>InternalResourceViewResolver</classname>,
the default list of <interfacename>ViewResolver</interfacename>
implementations is ignored.
</para>
<para>In <xref linkend="mvc-config"/> you'll learn about
other options for configuring Spring MVC including
MVC Java config and the MVC XML namespace both of which provide
a simple starting point and assume little knowledge of
how Spring MVC works. Regardless of how you choose to
configure your application, the concepts explained in this
section are fundamental should be of help to you.
</para>
</section>
<section xml:id="mvc-servlet-sequence">
<title>DispatcherServlet Processing Sequence</title>
<para>After you set up a <classname>DispatcherServlet</classname>, and a
request comes in for that specific
<classname>DispatcherServlet</classname>, the
<classname>DispatcherServlet</classname> starts processing the request as
follows:</para>
<orderedlist>
<listitem>
<para>The <interfacename>WebApplicationContext</interfacename> is
searched for and bound in the request as an attribute that the
controller and other elements in the process can use. <!--Use to do *what*? Also revise to indicate *what* searches for the WebApplicationContext -->It
is bound by default under the key
<literal>DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE</literal>.</para>
</listitem>
<listitem>
<para>The locale resolver is bound to the request to enable elements
in the process to resolve the locale to use when processing the
request (rendering the view, preparing data, and so on). If you do not
need locale resolving, you do not need it.</para>
<!--Reword 'if you don't need local resolving, you don't need to use it '. Are you saying locale resolving is optional? If you don't configure it, will this step occur?-->
</listitem>
<listitem>
<para>The theme resolver is bound to the request to let elements such
as views determine which theme to use. If you do not use themes, you
can ignore it.</para>
<!-- MLP perhaps say that there are not side effect to this binding, etc... Clarify *ignore it*. Does this step still occur if you don't use themes? -->
<!--And what if you DO use themes, what do you do and when? Same question re locale resolving.-->
</listitem>
<listitem>
<para>If you specify a multipart file resolver, the request is
inspected for multiparts; if multiparts are found, the request is
wrapped in a <classname>MultipartHttpServletRequest</classname> for
further processing by other elements in the process. See <xref
linkend="mvc-multipart" /> for further information about multipart
handling.</para>
</listitem>
<listitem>
<para>An appropriate handler is searched for. If a handler is found,
the execution chain associated with the handler (preprocessors,
postprocessors, and controllers) is executed in order to prepare a
model or rendering.</para>
</listitem>
<listitem>
<para>If a model is returned, the view is rendered. If no model is
returned, (may be due to a preprocessor or postprocessor intercepting
the request, perhaps for security reasons), no view is rendered,
because the request could already have been fulfilled.</para>
<!--fulfilled how and by what?-->
</listitem>
</orderedlist>
<para>Handler exception resolvers that are declared in the
<interfacename>WebApplicationContext</interfacename> pick up exceptions
that are thrown during processing of the request. Using these exception
resolvers allows you to define custom behaviors to address
exceptions.</para>
<para>The Spring <classname>DispatcherServlet</classname> also supports
the return of the <emphasis>last-modification-date</emphasis>, as
specified by the Servlet API. The process of determining the last
modification date for a specific request is straightforward: the
<classname>DispatcherServlet</classname> looks up an appropriate handler
mapping and tests whether the handler that is found implements the
<emphasis><interfacename>LastModified</interfacename></emphasis>
interface. If so, the value of the <literal>long
getLastModified(request)</literal> method of the
<interfacename>LastModified</interfacename> interface is returned to the
client.</para>
<para>You can customize individual
<classname>DispatcherServlet</classname> instances by adding Servlet
initialization parameters (<literal>init-param</literal> elements) to the
Servlet declaration in the <literal>web.xml</literal> file. See the
following table for the list of supported parameters.</para>
<!--Reword above sentence to specify whether configuring parameters in table configures last-modification-date, or are they further -->
<!--customization for some other purpose? If so, need to explain how you config last-modification-date-->
<table xml:id="mvc-disp-servlet-init-params-tbl">
<title><classname>DispatcherServlet</classname> initialization
parameters</title>
<tgroup cols="2">
<colspec colname="c1" colwidth="1*" />
<colspec colname="c2" colwidth="4*" />
<thead>
<row>
<entry>Parameter</entry>
<entry>Explanation</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>contextClass</literal></entry>
<entry>Class that implements
<interfacename>WebApplicationContext</interfacename>, which
instantiates the context used by this Servlet. By default, the
<classname>XmlWebApplicationContext</classname> is used.</entry>
</row>
<row>
<entry><literal>contextConfigLocation</literal></entry>
<entry>String that is passed to the context instance (specified by
<literal>contextClass</literal>) to indicate where context(s) can
be found. The string consists potentially of multiple strings
(using a comma as a delimiter) to support multiple contexts. In
case of multiple context locations with beans that are defined
twice, the latest location takes precedence.</entry>
<!-- MLP review -->
</row>
<row>
<entry><literal>namespace</literal></entry>
<entry>Namespace of the
<interfacename>WebApplicationContext</interfacename>. Defaults to
<literal>[servlet-name]-servlet</literal>.</entry>
</row>
</tbody>
</tgroup>
</table>
</section>
</section>
<section xml:id="mvc-controller">
<title>Implementing Controllers</title>
<para>Controllers provide access to the application behavior that you
typically define through a service interface. <!--I changed preceding to active voice because next sentence refers to user input. Thus *you* do some defining.-->Controllers
interpret user input and transform it into a model that is represented to
the user by the view. Spring implements a controller in a very abstract
way, which enables you to create a wide variety of controllers.</para>
<para>Spring 2.5 introduced an annotation-based programming model for MVC
controllers that uses annotations such as
<interfacename>@RequestMapping</interfacename>,
<interfacename>@RequestParam</interfacename>,
<interfacename>@ModelAttribute</interfacename>, and so on. This annotation
support is available for both Servlet MVC and Portlet MVC. Controllers
implemented in this style do not have to extend specific base classes or
implement specific interfaces. Furthermore, they do not usually have
direct dependencies on Servlet or Portlet APIs, although you can easily
configure access to Servlet or Portlet facilities.</para>
<tip>
<para>Available in the <link linkend="new-in-3.0-samples">samples
repository</link>, a number of web applications leverage the annotation
support described in this section including
<emphasis>MvcShowcase</emphasis>, <emphasis>MvcAjax</emphasis>,
<emphasis>MvcBasic</emphasis>, <emphasis>PetClinic</emphasis>,
<emphasis>PetCare</emphasis>, and others.</para>
<!-- MLP Note removed reference to imagedb -->
</tip>
<!--You need an intro sentence here that indicates the *purpose* of the following code. -->
<programlisting language="java">@Controller
public class HelloWorldController {
@RequestMapping("/helloWorld")
public String helloWorld(Model model) {
model.addAttribute("message", "Hello World!");
return "helloWorld";
}
}</programlisting>
<para>As you can see, the <interfacename>@Controller</interfacename> and
<interfacename>@RequestMapping</interfacename> annotations allow flexible
method names and signatures. In this particular example the method accepts
a <classname>Model</classname> and returns a view name as a
<classname>String</classname>, but various other method parameters and
return values can be used as explained later in this section.
<interfacename>@Controller</interfacename> and
<interfacename>@RequestMapping</interfacename> and a number of other
annotations form the basis for the Spring MVC implementation. This section
documents these annotations and how they are most commonly used in a
Servlet environment.</para>
<section xml:id="mvc-ann-controller">
<title>Defining a controller with
<interfacename>@Controller</interfacename></title>
<para>The <interfacename>@Controller</interfacename> annotation
indicates that a particular class serves the role of a
<emphasis>controller</emphasis>. Spring does not require you to extend
any controller base class or reference the Servlet API. However, you can
still reference Servlet-specific features if you need to.</para>
<para>The <interfacename>@Controller</interfacename> annotation acts as
a stereotype for the annotated class, indicating its role. The
dispatcher scans such annotated classes for mapped methods and detects
<interfacename>@RequestMapping</interfacename> annotations (see the next
section).</para>
<para>You can define annotated controller beans explicitly, using a
standard Spring bean definition in the dispatcher's context. However,
the <interfacename>@Controller</interfacename> stereotype also allows
for autodetection, aligned with Spring general support for detecting
component classes in the classpath and auto-registering bean definitions
for them.</para>
<!-- MLP Bev.changed to 'also supports autodetection -->
<para>To enable autodetection of such annotated controllers, you add
component scanning to your configuration. Use the
<emphasis>spring-context</emphasis> schema as shown in the following XML
snippet:</para>
<programlisting language="xml">&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd"&gt;
&lt;context:component-scan base-package="org.springframework.samples.petclinic.web"/&gt;
<lineannotation>&lt;!-- ... --&gt;</lineannotation>
&lt;/beans&gt;</programlisting>
</section>
<section xml:id="mvc-ann-requestmapping">
<title>Mapping Requests With
<interfacename>@RequestMapping</interfacename></title>
<para>You use the <interfacename>@RequestMapping</interfacename>
annotation to map URLs such as <filename>/appointments</filename> onto
an entire class or a particular handler method. Typically the
class-level annotation maps a specific request path (or path pattern)
onto a form controller, with additional method-level annotations
narrowing the primary mapping for a specific HTTP method request method
("GET", "POST", etc.) or an HTTP request parameter condition.</para>
<para>The following example from the <emphasis>Petcare</emphasis> sample
shows a controller in a Spring MVC application that uses this
annotation:</para>
<programlisting language="java">@Controller
<emphasis role="bold">@RequestMapping("/appointments")</emphasis>
public class AppointmentsController {
private final AppointmentBook appointmentBook;
@Autowired
public AppointmentsController(AppointmentBook appointmentBook) {
this.appointmentBook = appointmentBook;
}
<emphasis role="bold">@RequestMapping(method = RequestMethod.GET)</emphasis>
public Map&lt;String, Appointment&gt; get() {
return appointmentBook.getAppointmentsForToday();
}
<emphasis role="bold">@RequestMapping(value="/{day}", method = RequestMethod.GET)</emphasis>
public Map&lt;String, Appointment&gt; getForDay(@PathVariable @DateTimeFormat(iso=ISO.DATE) Date day, Model model) {
return appointmentBook.getAppointmentsForDay(day);
}
<emphasis role="bold">@RequestMapping(value="/new", method = RequestMethod.GET)</emphasis>
public AppointmentForm getNewForm() {
return new AppointmentForm();
}
<emphasis role="bold">@RequestMapping(method = RequestMethod.POST)</emphasis>
public String add(@Valid AppointmentForm appointment, BindingResult result) {
if (result.hasErrors()) {
return "appointments/new";
}
appointmentBook.addAppointment(appointment);
return "redirect:/appointments";
}
}</programlisting>
<para>In the example, the <interfacename>@RequestMapping</interfacename>
is used in a number of places. The first usage is on the type (class)
level, which indicates that all handling methods on this controller are
relative to the <filename>/appointments</filename> path. The
<methodname>get()</methodname> method has a further
<interfacename>@RequestMapping</interfacename> refinement: it only
accepts GET requests, meaning that an HTTP GET for
<filename>/appointments</filename> invokes this method. The
<methodname>post()</methodname> has a similar refinement, and the
<methodname>getNewForm()</methodname> combines the definition of HTTP
method and path into one, so that GET requests for
<filename>appointments/new</filename> are handled by that method.</para>
<para>The <methodname>getForDay()</methodname> method shows another
usage of <interfacename>@RequestMapping</interfacename>: URI templates.
(See <link linkend="mvc-ann-requestmapping-uri-templates">the next
section </link>).</para>
<para>A <interfacename>@RequestMapping</interfacename> on the class
level is not required. Without it, all paths are simply absolute, and
not relative. The following example from the
<emphasis>PetClinic</emphasis> sample application shows a multi-action
controller using <classname>@RequestMapping</classname>:</para>
<programlisting language="java">@Controller
public class ClinicController {
private final Clinic clinic;
@Autowired
public ClinicController(Clinic clinic) {
this.clinic = clinic;
}
<emphasis role="bold">@RequestMapping("/")</emphasis>
public void welcomeHandler() {
}
<emphasis role="bold">@RequestMapping("/vets")</emphasis>
public ModelMap vetsHandler() {
return new ModelMap(this.clinic.getVets());
}
}</programlisting>
<tip>
<title>Using <interfacename>@RequestMapping</interfacename> On
Interface Methods</title>
<para>A common pitfall when working with annotated controller classes
happens when applying functionality that requires creating a proxy for
the controller object (e.g.
<interfacename>@Transactional</interfacename> methods). Usually you
will introduce an interface for the controller in order to use JDK
dynamic proxies. To make this work you must move the
<interfacename>@RequestMapping</interfacename> annotations, as well as
any other type and method-level annotations (e.g.
<interfacename>@ModelAttribute</interfacename>,
<interfacename>@InitBinder</interfacename>) to the
interface as well as the mapping mechanism can only "see" the
interface exposed by the proxy. Alternatively, you could activate
<code>proxy-target-class="true"</code> in the configuration for the
functionality applied to the controller (in our transaction scenario
in <code>&lt;tx:annotation-driven /&gt;</code>). Doing so indicates
that CGLIB-based subclass proxies should be used instead of
interface-based JDK proxies. For more information on various proxying
mechanisms see <xref linkend="aop-proxying" />.</para>
<para>Note however that method argument annotations, e.g.
<interfacename>@RequestParam</interfacename>, must be present in
the method signatures of the controller class.</para>
</tip>
<section xml:id="mvc-ann-requestmapping-31-vs-30">
<title>New Support Classes for <classname>@RequestMapping</classname> methods in Spring MVC 3.1</title>
<para>Spring 3.1 introduced a new set of support classes for
<classname>@RequestMapping</classname> methods called
<classname>RequestMappingHandlerMapping</classname> and
<classname>RequestMappingHandlerAdapter</classname> respectively.
They are recommended for use and even required to take advantage of
new features in Spring MVC 3.1 and going forward. The new support
classes are enabled by default by the MVC namespace and the MVC Java
config but must be configured explicitly if using neither.
This section describes a few
important differences between the old and the new support classes.
</para>
<para>Prior to Spring 3.1, type and method-level request mappings were
examined in two separate stages -- a controller was selected first
by the <classname>DefaultAnnotationHandlerMapping</classname> and the
actual method to invoke was narrowed down second by
the <classname>AnnotationMethodHandlerAdapter</classname>.</para>
<para>With the new support classes in Spring 3.1, the
<classname>RequestMappingHandlerMapping</classname> is the only place
where a decision is made about which method should process the request.
Think of controller methods as a collection of unique endpoints
with mappings for each method derived from type and method-level
<classname>@RequestMapping</classname> information.</para>
<para>This enables some new possibilities. For once a
<classname>HandlerInterceptor</classname> or a
<classname>HandlerExceptionResolver</classname> can now expect the
Object-based handler to be a <classname>HandlerMethod</classname>,
which allows them to examine the exact method, its parameters and
associated annotations. The processing for a URL no longer needs to
be split across different controllers.
</para>
<para>There are also several things no longer possible:</para>
<itemizedlist>
<listitem><para>Select a controller first with a
<classname>SimpleUrlHandlerMapping</classname> or
<classname>BeanNameUrlHandlerMapping</classname> and then narrow
the method based on <classname>@RequestMapping</classname>
annotations.</para></listitem>
<listitem><para>Rely on method names as a fall-back mechanism to
disambiguate between two <classname>@RequestMapping</classname> methods
that don't have an explicit path mapping URL path but otherwise
match equally, e.g. by HTTP method. In the new support classes
<classname>@RequestMapping</classname> methods have to be mapped
uniquely.</para></listitem>
<listitem><para>Have a single default method (without an explicit
path mapping) with which requests are processed if no other
controller method matches more concretely. In the new support
classes if a matching method is not found a 404 error
is raised.</para></listitem>
</itemizedlist>
<para>The above features are still supported with the existing support
classes. However to take advantage of new Spring MVC 3.1 features
you'll need to use the new support classes.</para>
</section>
<section xml:id="mvc-ann-requestmapping-uri-templates">
<title>URI Template Patterns</title>
<para><emphasis>URI templates</emphasis> can be used for convenient
access to selected parts of a URL in a
<interfacename>@RequestMapping</interfacename> method.</para>
<para>A URI Template is a URI-like string, containing one or more
variable names. When you substitute values for these variables, the
template becomes a URI. The <link
xl:href="http://bitworking.org/projects/URI-Templates/">proposed
RFC</link> for URI Templates defines how a URI is parameterized. For
example, the URI Template
<code>http://www.example.com/users/{userId}</code> contains the
variable <emphasis>userId</emphasis>. Assigning the value
<emphasis>fred</emphasis> to the variable yields
<code>http://www.example.com/users/fred</code>.</para>
<para>In Spring MVC you can use the
<interfacename>@PathVariable</interfacename> annotation on a method
argument to bind it to the value of a URI template variable:</para>
<programlisting language="java">@RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET)
public String findOwner(<emphasis role="bold">@PathVariable</emphasis> String ownerId, Model model) {
Owner owner = ownerService.findOwner(ownerId);
model.addAttribute("owner", owner);
return "displayOwner";
}</programlisting>
<para>The URI Template "<literal>/owners/{ownerId}</literal>"
specifies the variable name <literal>ownerId</literal>. When the
controller handles this request, the value of
<literal>ownerId</literal> is set to the value found in the
appropriate part of the URI. For example, when a request comes in for
<code>/owners/fred</code>, the value of <literal>ownerId</literal> is
<literal>fred</literal>.</para>
<tip>
<para>To process the @PathVariable annotation, Spring MVC needs to
find the matching URI template variable by name. You can specify it
in the annotation:</para>
<programlisting language="java">@RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET)
public String findOwner(<emphasis role="bold">@PathVariable</emphasis>("ownerId") String theOwner, Model model) {
// implementation omitted
}</programlisting>
<para>Or if the URI template variable name matches the method
argument name you can omit that detail. As long as your code is not
compiled without debugging information, Spring MVC will match the
method argument name to the URI template variable name:</para>
<programlisting language="java">@RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET)
public String findOwner(<emphasis role="bold">@PathVariable</emphasis> String ownerId, Model model) {
// implementation omitted
}</programlisting>
</tip>
<para>A method can have any number of
<interfacename>@PathVariable</interfacename> annotations:</para>
<programlisting language="java">@RequestMapping(value="/owners/{ownerId}/pets/{petId}", method=RequestMethod.GET)
public String findPet(<emphasis role="bold">@PathVariable</emphasis> String ownerId, <emphasis
role="bold">@PathVariable</emphasis> String petId, Model model) {
Owner owner = ownerService.findOwner(ownerId);
Pet pet = owner.getPet(petId);
model.addAttribute("pet", pet);
return "displayPet";
}</programlisting>
<para>When a <interfacename>@PathVariable</interfacename> annotation is
used on a <classname>Map&lt;String, String&gt;</classname> argument, the
map is populated with all URI template variables.
</para>
<para>A URI template can be assembled from type and path level
<emphasis>@RequestMapping</emphasis> annotations. As a result the
<methodname>findPet()</methodname> method can be invoked with a URL
such as <filename>/owners/42/pets/21</filename>.</para>
<programlisting language="java">@Controller
@RequestMapping(<emphasis role="bold">"/owners/{ownerId}"</emphasis>)
public class RelativePathUriTemplateController {
@RequestMapping(<emphasis role="bold">"/pets/{petId}"</emphasis>)
public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
// implementation omitted
}
}</programlisting>
<para>A <interfacename>@PathVariable</interfacename> argument can be
of <emphasis role="bold">any simple type</emphasis> such as int, long,
Date, etc. Spring automatically converts to the appropriate type or
throws a <classname>TypeMismatchException</classname> if it fails to
do so. You can also register support for parsing additional data
types. See <xref linkend="mvc-ann-typeconversion" /> and <xref
linkend="mvc-ann-webdatabinder" />.</para>
</section>
<section xml:id="mvc-ann-requestmapping-uri-templates-regex">
<title>URI Template Patterns with Regular Expressions</title>
<para>Sometimes you need more precision in defining URI template
variables. Consider the URL
<code>"/spring-web/spring-web-3.0.5.jar"</code>. How do you break it
down into multiple parts?</para>
<para>The <interfacename>@RequestMapping</interfacename> annotation
supports the use of regular expressions in URI template variables. The
syntax is <code>{varName:regex}</code> where the first part defines
the variable name and the second - the regular expression.For
example:</para>
<programlisting language="java">
@RequestMapping("/spring-web/{symbolicName:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{extension:\\.[a-z]+}")
public void handle(@PathVariable String version, @PathVariable String extension) {
// ...
}
}</programlisting>
</section>
<section xml:id="mvc-ann-requestmapping-patterns">
<title>Path Patterns</title>
<para>In addition to URI templates, the
<interfacename>@RequestMapping</interfacename> annotation also
supports Ant-style path patterns (for example,
<filename>/myPath/*.do</filename>). A combination of URI templates and
Ant-style globs is also supported (for example,
<filename>/owners/*/pets/{petId}</filename>).</para>
</section>
<section xml:id="mvc-ann-requestmapping-placeholders">
<title>Patterns with Placeholders</title>
<para>Patterns in <interfacename>@RequestMapping</interfacename> annotations
support ${...} placeholders against local properties and/or system properties
and environment variables. This may be useful in cases where the path a
controller is mapped to may need to be customized through configuration.
For more information on placeholders see the Javadoc for
<classname>PropertyPlaceholderConfigurer</classname>.</para>
</section>
<section xml:id="mvc-ann-matrix-variables">
<title>Matrix Variables</title>
<para>The URI specification
<link xl:href="http://tools.ietf.org/html/rfc3986#section-3.3">RFC 3986</link>
defines the possibility of including name-value pairs within path segments.
There is no specific term used in the spec.
The general "URI path parameters" could be applied although the more unique
<link xl:href="http://www.w3.org/DesignIssues/MatrixURIs.html">"Matrix URIs"</link>,
originating from an old post by Tim Berners-Lee, is also frequently used
and fairly well known. Within Spring MVC these are referred to
as matrix variables.</para>
<para>Matrix variables can appear in any path segment, each matrix variable
separated with a ";" (semicolon).
For example: <code>"/cars;color=red;year=2012"</code>.
Multiple values may be either "," (comma) separated
<code>"color=red,green,blue"</code> or the variable name may be repeated
<code>"color=red;color=green;color=blue"</code>.</para>
<para>If a URL is expected to contain matrix variables, the request mapping
pattern must represent them with a URI template.
This ensures the request can be matched correctly regardless of whether
matrix variables are present or not and in what order they are
provided.</para>
<para>Below is an example of extracting the matrix variable "q":</para>
<programlisting language="java">// GET /pets/42;q=11;r=22
@RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET)
public void findPet(@PathVariable String petId, @MatrixVariable int q) {
// petId == 42
// q == 11
}</programlisting>
<para>Since all path segments may contain matrix variables, in some cases
you need to be more specific to identify where the variable is expected to be:</para>
<programlisting language="java">// GET /owners/42;q=11/pets/21;q=22
@RequestMapping(value = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)
public void findPet(
@MatrixVariable(value="q", pathVar="ownerId") int q1,
@MatrixVariable(value="q", pathVar="petId") int q2) {
// q1 == 11
// q2 == 22
}</programlisting>
<para>A matrix variable may be defined as optional and a default value specified:</para>
<programlisting language="java">// GET /pets/42
@RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET)
public void findPet(@MatrixVariable(required=true, defaultValue="1") int q) {
// q == 1
}</programlisting>
<para>All matrix variables may be obtained in a Map:</para>
<programlisting language="java">// GET /owners/42;q=11;r=12/pets/21;q=22;s=23
@RequestMapping(value = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)
public void findPet(
@MatrixVariable Map&lt;String, String&gt; matrixVars,
@MatrixVariable(pathVar="petId"") Map&lt;String, String&gt; petMatrixVars) {
// matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
// petMatrixVars: ["q" : 11, "s" : 23]
}</programlisting>
<para>Note that to enable the use of matrix variables, you must set the
<classname>removeSemicolonContent</classname> property of
<classname>RequestMappingHandlerMapping</classname> to <code>false</code>.
By default it is set to <code>true</code> with the exception of the
MVC namespace and the MVC Java config both of which automatically enable
the use of matrix variables.</para>
</section>
<section xml:id="mvc-ann-requestmapping-consumes">
<title>Consumable Media Types</title>
<para>You can narrow the primary mapping by specifying a list of
consumable media types. The request will be matched only if the
<emphasis>Content-Type</emphasis> request header matches the specified
media type. For example:</para>
<programlisting language="java">@Controller
@RequestMapping(value = "/pets", method = RequestMethod.POST, <emphasis
role="bold">consumes="application/json"</emphasis>)
public void addPet(@RequestBody Pet pet, Model model) {
// implementation omitted
}</programlisting>
<para>Consumable media type expressions can also be negated as in
<emphasis>!text/plain</emphasis> to match to all requests other than
those with <emphasis>Content-Type</emphasis> of
<emphasis>text/plain</emphasis>.</para>
<tip>
<para>The <emphasis>consumes</emphasis> condition is supported on
the type and on the method level. Unlike most other conditions, when
used at the type level, method-level consumable types override
rather than extend type-level consumable types.</para>
</tip>
</section>
<section xml:id="mvc-ann-requestmapping-produces">
<title>Producible Media Types</title>
<para>You can narrow the primary mapping by specifying a list of
producible media types. The request will be matched only if the
<emphasis>Accept</emphasis> request header matches one of these
values. Furthermore, use of the <emphasis>produces</emphasis>
condition ensures the actual content type used to generate the
response respects the media types specified in the
<emphasis>produces</emphasis> condition. For example:</para>
<programlisting language="java">@Controller
@RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET, <emphasis
role="bold">produces="application/json"</emphasis>)
@ResponseBody
public Pet getPet(@PathVariable String petId, Model model) {
// implementation omitted
}</programlisting>
<para>Just like with <emphasis>consumes</emphasis>, producible media
type expressions can be negated as in <emphasis>!text/plain</emphasis>
to match to all requests other than those with an
<emphasis>Accept</emphasis> header value of
<emphasis>text/plain</emphasis>.</para>
<tip>
<para>The <emphasis>produces</emphasis> condition is supported on
the type and on the method level. Unlike most other conditions, when
used at the type level, method-level producible types override
rather than extend type-level producible types.</para>
</tip>
</section>
<section xml:id="mvc-ann-requestmapping-params-and-headers">
<title>Request Parameters and Header Values</title>
<para>You can narrow request matching through request parameter
conditions such as <code>"myParam"</code>, <code>"!myParam"</code>, or
<code>"myParam=myValue"</code>. The first two test for request
parameter presence/absence and the third for a specific parameter
value. Here is an example with a request parameter value
condition:</para>
<programlisting language="java">@Controller
@RequestMapping("/owners/{ownerId}")
public class RelativePathUriTemplateController {
@RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET, <emphasis
role="bold">params="myParam=myValue"</emphasis>)
public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
// implementation omitted
}
}</programlisting>
<para>The same can be done to test for request header presence/absence
or to match based on a specific request header value:</para>
<programlisting language="java">@Controller
@RequestMapping("/owners/{ownerId}")
public class RelativePathUriTemplateController {
@RequestMapping(value = "/pets", method = RequestMethod.GET, <emphasis
role="bold">headers="myHeader=myValue"</emphasis>)
public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
// implementation omitted
}
}</programlisting>
<tip>
<para>Although you can match to <emphasis>Content-Type</emphasis>
and <emphasis>Accept</emphasis> header values using media type wild
cards (for example <emphasis>"content-type=text/*"</emphasis> will
match to <emphasis>"text/plain"</emphasis> and
<emphasis>"text/html"</emphasis>), it is recommended to use the
<emphasis>consumes</emphasis> and <emphasis>produces</emphasis>
conditions respectively instead. They are intended specifically for
that purpose.</para>
</tip>
</section>
</section>
<section xml:id="mvc-ann-methods">
<title>Defining <interfacename>@RequestMapping</interfacename> handler
methods</title>
<para>An <classname>@RequestMapping</classname> handler method can have
a very flexible signatures. The supported method arguments and return
values are described in the following section. Most arguments can be
used in arbitrary order with the only exception of
<classname>BindingResult</classname> arguments. This is described in the
next section.</para>
<note><para>Spring 3.1 introduced a new set of support classes for
<classname>@RequestMapping</classname> methods called
<classname>RequestMappingHandlerMapping</classname> and
<classname>RequestMappingHandlerAdapter</classname> respectively.
They are recommended for use and even required to take advantage
of new features in Spring MVC 3.1 and going forward.
The new support classes are enabled by default from the MVC namespace and
with use of the MVC Java config but must be
configured explicitly if using neither.
</para></note>
<section xml:id="mvc-ann-arguments">
<title>Supported method argument types</title>
<para>The following are the supported method arguments: <itemizedlist>
<listitem>
<para>Request or response objects (Servlet API). Choose any
specific request or response type, for example
<interfacename>ServletRequest</interfacename> or
<interfacename>HttpServletRequest</interfacename>.</para>
</listitem>
<listitem>
<para>Session object (Servlet API): of type
<interfacename>HttpSession</interfacename>. An argument of this
type enforces the presence of a corresponding session. As a
consequence, such an argument is never
<literal>null</literal>.</para>
<note>
<para>Session access may not be thread-safe, in particular in
a Servlet environment. Consider setting the
<classname>RequestMappingHandlerAdapter</classname>'s
"synchronizeOnSession" flag to "true" if multiple requests are
allowed to access a session concurrently.</para>
</note>
</listitem>
<listitem>
<para><classname>org.springframework.web.context.request.WebRequest</classname>
or
<classname>org.springframework.web.context.request.NativeWebRequest</classname>.
Allows for generic request parameter access as well as
request/session attribute access, without ties to the native
Servlet/Portlet API.</para>
</listitem>
<listitem>
<para><classname>java.util.Locale</classname> for the current
request locale, determined by the most specific locale resolver
available, in effect, the configured
<interfacename>LocaleResolver</interfacename> in a Servlet
environment.</para>
</listitem>
<listitem>
<para><classname>java.io.InputStream</classname> /
<classname>java.io.Reader</classname> for access to the
request's content. This value is the raw InputStream/Reader as
exposed by the Servlet API.</para>
</listitem>
<listitem>
<para><classname>java.io.OutputStream</classname> /
<classname>java.io.Writer</classname> for generating the
response's content. This value is the raw OutputStream/Writer as
exposed by the Servlet API.</para>
</listitem>
<listitem>
<para><interfacename>java.security.Principal</interfacename>
containing the currently authenticated user.</para>
</listitem>
<listitem>
<para><classname>@PathVariable</classname> annotated parameters
for access to URI template variables. See <xref
linkend="mvc-ann-requestmapping-uri-templates" />.</para>
</listitem>
<listitem>
<para><classname>@MatrixVariable</classname> annotated parameters
for access to name-value pairs located in URI path segments.
See <xref linkend="mvc-ann-matrix-variables" />.</para>
</listitem>
<listitem>
<para><classname>@RequestParam</classname> annotated parameters
for access to specific Servlet request parameters. Parameter
values are converted to the declared method argument type. See
<xref linkend="mvc-ann-requestparam" />.</para>
</listitem>
<listitem>
<para><interfacename>@RequestHeader</interfacename> annotated
parameters for access to specific Servlet request HTTP headers.
Parameter values are converted to the declared method argument
type.</para>
</listitem>
<listitem>
<para><interfacename>@RequestBody</interfacename> annotated
parameters for access to the HTTP request body. Parameter values
are converted to the declared method argument type using
<interfacename>HttpMessageConverter</interfacename>s. See <xref
linkend="mvc-ann-requestbody" />.</para>
</listitem>
<listitem>
<para><interfacename>@RequestPart</interfacename> annotated
parameters for access to the content of a "multipart/form-data"
request part. See <xref
linkend="mvc-multipart-forms-non-browsers" /> and <xref
linkend="mvc-multipart" />.</para>
</listitem>
<listitem>
<para><classname>HttpEntity&lt;?&gt;</classname> parameters for
access to the Servlet request HTTP headers and contents. The
request stream will be converted to the entity body using
<interfacename>HttpMessageConverter</interfacename>s. See <xref
linkend="mvc-ann-httpentity" />.</para>
</listitem>
<listitem>
<para><interfacename>java.util.Map</interfacename> /
<interfacename>org.springframework.ui.Model</interfacename> /
<classname>org.springframework.ui.ModelMap</classname> for
enriching the implicit model that is exposed to the web
view.</para>
</listitem>
<listitem>
<para><interfacename>org.springframework.web.servlet.mvc.support.RedirectAttributes</interfacename>
to specify the exact set of attributes to use in case of a
redirect and also to add flash attributes (attributes stored
temporarily on the server-side to make them available to the
request after the redirect).
<literal>RedirectAttributes</literal> is used instead of the
implicit model if the method returns a "redirect:" prefixed view
name or <classname>RedirectView</classname>.</para>
</listitem>
<listitem>
<para>Command or form objects to bind request parameters to bean
properties (via setters) or directly to fields, with
customizable type conversion, depending on
<classname>@InitBinder</classname> methods and/or the
HandlerAdapter configuration. See the
<literal>webBindingInitializer</literal> property on
<classname>RequestMappingHandlerAdapter</classname>. Such
command objects along with their validation results will be
exposed as model attributes by default, using the command class
class name - e.g. model attribute "orderAddress" for a command
object of type "some.package.OrderAddress". The
<classname>ModelAttribute</classname> annotation can be used on
a method argument to customize the model attribute name
used.</para>
</listitem>
<listitem>
<para><classname>org.springframework.validation.Errors</classname>
/
<classname>org.springframework.validation.BindingResult</classname>
validation results for a preceding command or form object (the
immediately preceding method argument).</para>
</listitem>
<listitem>
<para><classname>org.springframework.web.bind.support.SessionStatus</classname>
status handle for marking form processing as complete, which
triggers the cleanup of session attributes that have been
indicated by the <classname>@SessionAttributes</classname>
annotation at the handler type level.</para>
</listitem>
<listitem>
<para><classname>org.springframework.web.util.UriComponentsBuilder</classname>
a builder for preparing a URL relative to the current request's
host, port, scheme, context path, and the literal part of the
servlet mapping.</para>
</listitem>
</itemizedlist></para>
<para>The <interfacename>Errors</interfacename> or
<interfacename>BindingResult</interfacename> parameters have to follow
the model object that is being bound immediately as the method
signature might have more that one model object and Spring will create
a separate <interfacename>BindingResult</interfacename> instance for
each of them so the following sample won't work:</para>
<example>
<title>Invalid ordering of BindingResult and @ModelAttribute</title>
<programlisting language="java">@RequestMapping(method = RequestMethod.POST)
public String processSubmit(<emphasis role="bold">@ModelAttribute("pet") Pet pet</emphasis>,
Model model, <emphasis role="bold">BindingResult result</emphasis>) { … }</programlisting>
<para>Note, that there is a <interfacename>Model</interfacename>
parameter in between <classname>Pet</classname> and
<interfacename>BindingResult</interfacename>. To get this working
you have to reorder the parameters as follows:</para>
<programlisting language="java">@RequestMapping(method = RequestMethod.POST)
public String processSubmit(<emphasis role="bold">@ModelAttribute("pet") Pet pet</emphasis>,
<emphasis role="bold">BindingResult result</emphasis>, Model model) { … }</programlisting>
</example>
</section>
<section xml:id="mvc-ann-return-types">
<title>Supported method return types</title>
<para>The following are the supported return types: <itemizedlist>
<listitem>
<para>A <classname>ModelAndView</classname> object, with the
model implicitly enriched with command objects and the results
of <literal>@ModelAttribute</literal> annotated reference data
accessor methods.</para>
</listitem>
<listitem>
<para>A <interfacename>Model</interfacename> object, with the
view name implicitly determined through a
<interfacename>RequestToViewNameTranslator</interfacename> and
the model implicitly enriched with command objects and the
results of <literal>@ModelAttribute</literal> annotated
reference data accessor methods.</para>
</listitem>
<listitem>
<para>A <interfacename>Map</interfacename> object for exposing a
model, with the view name implicitly determined through a
<interfacename>RequestToViewNameTranslator</interfacename> and
the model implicitly enriched with command objects and the
results of <literal>@ModelAttribute</literal> annotated
reference data accessor methods.</para>
</listitem>
<listitem>
<para>A <interfacename>View</interfacename> object, with the
model implicitly determined through command objects and
<literal>@ModelAttribute</literal> annotated reference data
accessor methods. The handler method may also programmatically
enrich the model by declaring a
<interfacename>Model</interfacename> argument (see above).<!--see above where? Need more explicit reference. same problem with next item.--></para>
</listitem>
<listitem>
<para>A <classname>String</classname> value that is interpreted
as the logical view name, with the model implicitly determined
through command objects and <literal>@ModelAttribute</literal>
annotated reference data accessor methods. The handler method
may also programmatically enrich the model by declaring a
<interfacename>Model</interfacename> argument (see
above).</para>
</listitem>
<listitem>
<para><literal>void</literal> if the method handles the response
itself (by writing the response content directly, declaring an
argument of type <interfacename>ServletResponse</interfacename>
/ <interfacename>HttpServletResponse</interfacename> for that
purpose) or if the view name is supposed to be implicitly
determined through a
<interfacename>RequestToViewNameTranslator</interfacename> (not
declaring a response argument in the handler method
signature).</para>
</listitem>
<listitem>
<para>If the method is annotated with
<interfacename>@ResponseBody</interfacename>, the return type is
written to the response HTTP body. The return value will be
converted to the declared method argument type using
<interfacename>HttpMessageConverter</interfacename>s. See <xref
linkend="mvc-ann-responsebody" />.</para>
</listitem>
<listitem>
<para>A <classname>HttpEntity&lt;?&gt;</classname> or
<classname>ResponseEntity&lt;?&gt;</classname> object to provide
access to the Servlet response HTTP headers and contents. The
entity body will be converted to the response stream using
<interfacename>HttpMessageConverter</interfacename>s. See <xref
linkend="mvc-ann-httpentity" />.</para>
</listitem>
<listitem>
<para>A <interfacename>Callable&lt;?&gt;</interfacename> can
be returned when the application wants to produce the return
value asynchronously in a thread managed by Spring MVC.</para>
</listitem>
<listitem>
<para>A <classname>DeferredResult&lt;?&gt;</classname> can
be returned when the application wants to produce the return
value from a thread of its own choosing.</para>
</listitem>
<listitem>
<para>Any other return type is considered to be a single model
attribute to be exposed to the view, using the attribute name
specified through <literal>@ModelAttribute</literal> at the
method level (or the default attribute name based on the return
type class name). The model is implicitly enriched with command
objects and the results of <literal>@ModelAttribute</literal>
annotated reference data accessor methods.</para>
</listitem>
</itemizedlist></para>
</section>
<section xml:id="mvc-ann-requestparam">
<title>Binding request parameters to method parameters with
<interfacename>@RequestParam</interfacename></title>
<para>Use the <classname>@RequestParam</classname> annotation to bind
request parameters to a method parameter in your controller.</para>
<para>The following code snippet shows the usage:</para>
<programlisting language="java">@Controller
@RequestMapping("/pets")
@SessionAttributes("pet")
public class EditPetForm {
<lineannotation>// ...</lineannotation>
@RequestMapping(method = RequestMethod.GET)
public String setupForm(<emphasis role="bold">@RequestParam("petId") int petId</emphasis>, ModelMap model) {
Pet pet = this.clinic.loadPet(petId);
model.addAttribute("pet", pet);
return "petForm";
}
<lineannotation>// ...</lineannotation></programlisting>
<para>Parameters using this annotation are required by default, but
you can specify that a parameter is optional by setting
<interfacename>@RequestParam</interfacename>'s
<literal>required</literal> attribute to <literal>false</literal>
(e.g., <literal>@RequestParam(value="id",
required=false)</literal>).</para>
<para>Type conversion is applied automatically if the target method
parameter type is not <classname>String</classname>. See <xref
linkend="mvc-ann-typeconversion" />.</para>
</section>
<section xml:id="mvc-ann-requestbody">
<title>Mapping the request body with the @RequestBody
annotation</title>
<para>The <classname>@RequestBody</classname> method parameter
annotation indicates that a method parameter should be bound to the
value of the HTTP request body. For example:</para>
<programlisting language="java">@RequestMapping(value = "/something", method = RequestMethod.PUT)
public void handle(@RequestBody String body, Writer writer) throws IOException {
writer.write(body);
}</programlisting>
<para>You convert the request body to the method argument by using an
<interfacename>HttpMessageConverter</interfacename>.
<interfacename>HttpMessageConverter</interfacename> is responsible for
converting from the HTTP request message to an object and converting
from an object to the HTTP response body. The
<classname>RequestMappingHandlerAdapter</classname> supports the
<classname>@RequestBody</classname> annotation with the following
default <interfacename>HttpMessageConverters</interfacename>:</para>
<itemizedlist>
<listitem>
<para><classname>ByteArrayHttpMessageConverter</classname>
converts byte arrays.</para>
</listitem>
<listitem>
<para><classname>StringHttpMessageConverter</classname> converts
strings.</para>
</listitem>
<listitem>
<para><classname>FormHttpMessageConverter</classname> converts
form data to/from a MultiValueMap&lt;String, String&gt;.</para>
</listitem>
<listitem>
<para><classname>SourceHttpMessageConverter</classname> converts
to/from a javax.xml.transform.Source.</para>
</listitem>
</itemizedlist>
<para>For more information on these converters, see <link
linkend="rest-message-conversion">Message Converters</link>. Also note
that if using the MVC namespace or the MVC Java config, a wider
range of message converters are registered by default.
See <link linkend="mvc-config-enable">Enabling the MVC Java Config or
the MVC XML Namespace</link> for more information.</para>
<para>If you intend to read and write XML, you will need to configure
the <classname>MarshallingHttpMessageConverter</classname> with a
specific <interfacename>Marshaller</interfacename> and an
<interfacename>Unmarshaller</interfacename> implementation from the
<classname>org.springframework.oxm</classname> package. The example
below shows how to do that directly in your configuration but if
your application is configured through the MVC namespace or the
MVC Java config see <link linkend="mvc-config-enable">Enabling
the MVC Java Config or the MVC XML Namespace</link> instead.</para>
<programlisting language="xml">&lt;bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"&gt;
&lt;property name="messageConverters"&gt;
&lt;util:list id="beanList"&gt;
&lt;ref bean="stringHttpMessageConverter"/&gt;
&lt;ref bean="marshallingHttpMessageConverter"/&gt;
&lt;/util:list&gt;
&lt;/property
&lt;/bean&gt;
&lt;bean id="stringHttpMessageConverter"
class="org.springframework.http.converter.StringHttpMessageConverter"/&gt;
&lt;bean id="marshallingHttpMessageConverter"
class="org.springframework.http.converter.xml.MarshallingHttpMessageConverter"&gt;
&lt;property name="marshaller" ref="castorMarshaller" /&gt;
&lt;property name="unmarshaller" ref="castorMarshaller" /&gt;
&lt;/bean&gt;
&lt;bean id="castorMarshaller" class="org.springframework.oxm.castor.CastorMarshaller"/&gt;</programlisting>
<para>An <classname>@RequestBody</classname> method parameter can be
annotated with <classname>@Valid</classname>, in which case it will be
validated using the configured <classname>Validator</classname>
instance. When using the MVC namespace or the MVC Java config, a JSR-303 validator
is configured automatically assuming a JSR-303 implementation is
available on the classpath.</para>
<para>Just like with <classname>@ModelAttribute</classname> parameters,
an <classname>Errors</classname> argument can be used to examine the errors.
If such an argument is not declared, a
<classname>MethodArgumentNotValidException</classname> will be raised.
The exception is handled in the
<classname>DefaultHandlerExceptionResolver</classname>, which sends
a <literal>400</literal> error back to the client.</para>
<note>
<para>Also see <link linkend="mvc-config-enable">Enabling the MVC
Java Config or the MVC XML Namespace</link> for
information on configuring message converters and a validator
through the MVC namespace or the MVC Java config.</para>
</note>
</section>
<section xml:id="mvc-ann-responsebody">
<title>Mapping the response body with the
<interfacename>@ResponseBody</interfacename> annotation</title>
<para>The <interfacename>@ResponseBody</interfacename> annotation is
similar to <interfacename>@RequestBody</interfacename>. This
annotation can be put on a method and indicates that the return type
should be written straight to the HTTP response body (and not placed
in a Model, or interpreted as a view name). For example:</para>
<programlisting language="java">@RequestMapping(value = "/something", method = RequestMethod.PUT)
@ResponseBody
public String helloWorld() {
return "Hello World";
}</programlisting>
<para>The above example will result in the text <literal>Hello
World</literal> being written to the HTTP response stream.</para>
<para>As with <interfacename>@RequestBody</interfacename>, Spring
converts the returned object to a response body by using an
<interfacename>HttpMessageConverter</interfacename>. For more
information on these converters, see the previous section and <link
linkend="rest-message-conversion">Message Converters</link>.</para>
</section>
<section xml:id="mvc-ann-httpentity">
<title>Using <classname>HttpEntity&lt;?&gt;</classname></title>
<para>The <classname>HttpEntity</classname> is similar to
<interfacename>@RequestBody</interfacename> and
<interfacename>@ResponseBody</interfacename>. Besides getting access
to the request and response body, <classname>HttpEntity</classname>
(and the response-specific subclass
<classname>ResponseEntity</classname>) also allows access to the
request and response headers, like so:</para>
<programlisting language="java">@RequestMapping("/something")
public ResponseEntity&lt;String&gt; handle(HttpEntity&lt;byte[]&gt; requestEntity) throws UnsupportedEncodingException {
String requestHeader = requestEntity.getHeaders().getFirst("MyRequestHeader"));
byte[] requestBody = requestEntity.getBody();
// do something with request header and body
HttpHeaders responseHeaders = new HttpHeaders();
responseHeaders.set("MyResponseHeader", "MyValue");
return new ResponseEntity&lt;String&gt;("Hello World", responseHeaders, HttpStatus.CREATED);
}</programlisting>
<para>The above example gets the value of the
<literal>MyRequestHeader</literal> request header, and reads the body
as a byte array. It adds the <literal>MyResponseHeader</literal> to
the response, writes <literal>Hello World</literal> to the response
stream, and sets the response status code to 201 (Created).</para>
<para>As with <interfacename>@RequestBody</interfacename> and
<interfacename>@ResponseBody</interfacename>, Spring uses
<interfacename>HttpMessageConverter</interfacename> to convert from
and to the request and response streams. For more information on these
converters, see the previous section and <link
linkend="rest-message-conversion">Message Converters</link>.</para>
</section>
<section xml:id="mvc-ann-modelattrib-methods">
<title>Using <interfacename>@ModelAttribute</interfacename> on a
method</title>
<para>The <interfacename>@ModelAttribute</interfacename> annotation
can be used on methods or on method arguments. This section explains
its usage on methods while the next section explains its usage on
method arguments.</para>
<para>An <interfacename>@ModelAttribute</interfacename> on a method
indicates the purpose of that method is to add one or more model
attributes. Such methods support the same argument types as
<interfacename>@RequestMapping</interfacename> methods but cannot be
mapped directly to requests. Instead
<interfacename>@ModelAttribute</interfacename> methods in a controller
are invoked before <interfacename>@RequestMapping</interfacename>
methods, within the same controller. A couple of examples:</para>
<programlisting language="java">
// Add one attribute
// The return value of the method is added to the model under the name "account"
// You can customize the name via @ModelAttribute("myAccount")
@ModelAttribute
public Account addAccount(@RequestParam String number) {
return accountManager.findAccount(number);
}
// Add multiple attributes
@ModelAttribute
public void populateModel(@RequestParam String number, Model model) {
model.addAttribute(accountManager.findAccount(number));
// add more ...
}</programlisting>
<para><interfacename>@ModelAttribute</interfacename> methods are used
to populate the model with commonly needed attributes for example to
fill a drop-down with states or with pet types, or to retrieve a
command object like Account in order to use it to represent the data
on an HTML form. The latter case is further discussed in the next
section.</para>
<para>Note the two styles of
<interfacename>@ModelAttribute</interfacename> methods. In the first,
the method adds an attribute implicitly by returning it. In the
second, the method accepts a <classname>Model</classname> and adds any
number of model attributes to it. You can choose between the two
styles depending on your needs.</para>
<para>A controller can have any number of
<interfacename>@ModelAttribute</interfacename> methods. All such
methods are invoked before
<interfacename>@RequestMapping</interfacename> methods of the same
controller.</para>
<para><interfacename>@ModelAttribute</interfacename> methods can also
be defined in an <interfacename>@ControllerAdvice</interfacename>-annotated
class and such methods apply to all controllers.
The <interfacename>@ControllerAdvice</interfacename> annotation is
a component annotation allowing implementation classes to be autodetected
through classpath scanning.</para>
<tip>
<para>What happens when a model attribute name is not explicitly
specified? In such cases a default name is assigned to the model
attribute based on its type. For example if the method returns an
object of type <classname>Account</classname>, the default name used
is "account". You can change that through the value of the
<interfacename>@ModelAttribute</interfacename> annotation. If adding
attributes directly to the <classname>Model</classname>, use the
appropriate overloaded <literal>addAttribute(..)</literal> method -
i.e., with or without an attribute name.</para>
</tip>
<para>The <interfacename>@ModelAttribute</interfacename> annotation
can be used on <interfacename>@RequestMapping</interfacename> methods
as well. In that case the return value of the
<interfacename>@RequestMapping</interfacename> method is interpreted
as a model attribute rather than as a view name. The view name is
derived from view name conventions instead much like for methods
returning void — see <xref linkend="mvc-coc-r2vnt" />.</para>
</section>
<section xml:id="mvc-ann-modelattrib-method-args">
<title>Using <interfacename>@ModelAttribute</interfacename> on a
method argument</title>
<para>As explained in the previous section
<interfacename>@ModelAttribute</interfacename> can be used on methods
or on method arguments. This section explains its usage on method
arguments.</para>
<para>An <interfacename>@ModelAttribute</interfacename> on a method
argument indicates the argument should be retrieved from the model. If
not present in the model, the argument should be instantiated first
and then added to the model. Once present in the model, the argument's
fields should be populated from all request parameters that have
matching names. This is known as data binding in Spring MVC, a very
useful mechanism that saves you from having to parse each form field
individually.</para>
<programlisting language="java">
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
public String processSubmit(<emphasis role="bold">@ModelAttribute Pet pet</emphasis>) {
}</programlisting>
<para>Given the above example where can the Pet instance come from?
There are several options:</para>
<itemizedlist>
<listitem>
<para>It may already be in the model due to use of
<interfacename>@SessionAttributes</interfacename> — see <xref
linkend="mvc-ann-sessionattrib" />.</para>
</listitem>
<listitem>
<para>It may already be in the model due to an
<interfacename>@ModelAttribute</interfacename> method in the same
controller — as explained in the previous section.</para>
</listitem>
<listitem>
<para>It may be retrieved based on a URI template variable and
type converter (explained in more detail below).</para>
</listitem>
<listitem>
<para>It may be instantiated using its default constructor.</para>
</listitem>
</itemizedlist>
<para>An <interfacename>@ModelAttribute</interfacename> method is a
common way to to retrieve an attribute from the database, which may
optionally be stored between requests through the use of
<interfacename>@SessionAttributes</interfacename>. In some cases it
may be convenient to retrieve the attribute by using an URI template
variable and a type converter. Here is an example:</para>
<programlisting language="java">
@RequestMapping(value="/accounts/{account}", method = RequestMethod.PUT)
public String save(@ModelAttribute("account") Account account) {
}</programlisting>
<para>In this example the name of the model attribute (i.e. "account")
matches the name of a URI template variable. If you register
<classname>Converter&lt;String, Account&gt;</classname> that can turn
the <literal>String</literal> account value into an
<classname>Account</classname> instance, then the above example will
work without the need for an
<interfacename>@ModelAttribute</interfacename> method.</para>
<para>The next step is data binding. The
<classname>WebDataBinder</classname> class matches request parameter
names — including query string parameters and form fields — to model
attribute fields by name. Matching fields are populated after type
conversion (from String to the target field type) has been applied
where necessary. Data binding and validation are covered in <xref
linkend="validation" />. Customizing the data binding process for a
controller level is covered in <xref
linkend="mvc-ann-webdatabinder" />.</para>
<para>As a result of data binding there may be errors such as missing
required fields or type conversion errors. To check for such errors
add a <classname>BindingResult</classname> argument immediately
following the <interfacename>@ModelAttribute</interfacename>
argument:</para>
<programlisting language="java">
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
public String processSubmit(<emphasis role="bold">@ModelAttribute("pet") Pet pet</emphasis>, BindingResult result) {
if (result.hasErrors()) {
return "petForm";
}
// ...
}</programlisting>
<para>With a <classname>BindingResult</classname> you can check if
errors were found in which case it's common to render the same form
where the errors can be shown with the help of Spring's
<literal>&lt;errors&gt;</literal> form tag.</para>
<para>In addition to data binding you can also invoke validation using
your own custom validator passing the same
<classname>BindingResult</classname> that was used to record data
binding errors. That allows for data binding and validation errors to
be accumulated in one place and subsequently reported back to the
user:</para>
<programlisting language="java">
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
public String processSubmit(<emphasis role="bold">@ModelAttribute("pet") Pet pet</emphasis>, BindingResult result) {
new PetValidator().validate(pet, result);
if (result.hasErrors()) {
return "petForm";
}
// ...
}</programlisting>
<para>Or you can have validation invoked automatically by adding the
JSR-303 <interfacename>@Valid</interfacename> annotation:</para>
<programlisting language="java">
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)
public String processSubmit(<emphasis role="bold">@Valid @ModelAttribute("pet") Pet pet</emphasis>, BindingResult result) {
if (result.hasErrors()) {
return "petForm";
}
// ...
}</programlisting>
<para>See <xref linkend="validation-beanvalidation" /> and <xref
linkend="validation" /> for details on how to configure and use
validation.</para>
</section>
<section xml:id="mvc-ann-sessionattrib">
<title>Using <classname>@SessionAttributes</classname> to store model
attributes in the HTTP session between requests</title>
<para>The type-level <classname>@SessionAttributes</classname>
annotation declares session attributes used by a specific handler.
This will typically list the names of model attributes or types of
model attributes which should be transparently stored in the session
or some conversational storage, serving as form-backing beans between
subsequent requests.</para>
<para>The following code snippet shows the usage of this annotation,
specifying the model attribute name:</para>
<programlisting language="java">@Controller
@RequestMapping("/editPet.do")
<emphasis role="bold">@SessionAttributes("pet")</emphasis>
public class EditPetForm {
<lineannotation>// ...</lineannotation>
}</programlisting>
<note>
<para>When using controller interfaces (e.g., for AOP proxying),
make sure to consistently put <emphasis>all</emphasis> your mapping
annotations - such as <interfacename>@RequestMapping</interfacename>
and <interfacename>@SessionAttributes</interfacename> - on the
controller <emphasis>interface</emphasis> rather than on the
implementation class.</para>
</note>
</section>
<section xml:id="mvc-ann-redirect-attributes">
<title>Specifying redirect and flash attributes</title>
<para>By default all model attributes are considered to be exposed as
URI template variables in the redirect URL. Of the remaining
attributes those that are primitive types or collections/arrays of
primitive types are automatically appended as query parameters.</para>
<para>In annotated controllers however the model may contain
additional attributes originally added for rendering purposes (e.g.
drop-down field values). To gain precise control over the attributes
used in a redirect scenario, an
<interfacename>@RequestMapping</interfacename> method can declare an
argument of type <interfacename>RedirectAttributes</interfacename> and
use it to add attributes for use in
<classname>RedirectView</classname>. If the controller method does
redirect, the content of
<interfacename>RedirectAttributes</interfacename> is used. Otherwise
the content of the default <interfacename>Model</interfacename> is
used.</para>
<para>The <classname>RequestMappingHandlerAdapter</classname> provides
a flag called <literal>"ignoreDefaultModelOnRedirect"</literal> that
can be used to indicate the content of the default
<interfacename>Model</interfacename> should never be used if a
controller method redirects. Instead the controller method should
declare an attribute of type
<interfacename>RedirectAttributes</interfacename> or if it doesn't do
so no attributes should be passed on to
<classname>RedirectView</classname>. Both the MVC namespace and the
MVC Java config keep this flag set to <literal>false</literal> in order to maintain
backwards compatibility. However, for new applications we recommend
setting it to <literal>true</literal></para>
<para>The <interfacename>RedirectAttributes</interfacename> interface
can also be used to add flash attributes. Unlike other redirect
attributes, which end up in the target redirect URL, flash attributes
are saved in the HTTP session (and hence do not appear in the URL).
The model of the controller serving the target redirect URL
automatically receives these flash attributes after which they are
removed from the session. See <xref linkend="mvc-flash-attributes" />
for an overview of the general support for flash attributes in Spring
MVC.</para>
</section>
<section xml:id="mvc-ann-form-urlencoded-data">
<title>Working with
<literal>"application/x-www-form-urlencoded"</literal> data</title>
<para>The previous sections covered use of
<interfacename>@ModelAttribute</interfacename> to support form
submission requests from browser clients. The same annotation is
recommended for use with requests from non-browser clients as well.
However there is one notable difference when it comes to working with
HTTP PUT requests. Browsers can submit form data via HTTP GET or HTTP
POST. Non-browser clients can also submit forms via HTTP PUT. This
presents a challenge because the Servlet specification requires the
<literal>ServletRequest.getParameter*()</literal> family of methods to
support form field access only for HTTP POST, not for HTTP PUT.</para>
<para>To support HTTP PUT and PATCH requests, the <literal>spring-web</literal>
module provides the filter
<classname>HttpPutFormContentFilter</classname>, which can be
configured in <filename>web.xml</filename>:</para>
<programlisting language="xml">&lt;filter&gt;
&lt;filter-name&gt;httpPutFormFilter&lt;/filter-name&gt;
&lt;filter-class&gt;org.springframework.web.filter.HttpPutFormContentFilter&lt;/filter-class&gt;
&lt;/filter&gt;
&lt;filter-mapping&gt;
&lt;filter-name&gt;httpPutFormFilter&lt;/filter-name&gt;
&lt;servlet-name&gt;dispatcherServlet&lt;/servlet-name&gt;
&lt;/filter-mapping&gt;
&lt;servlet&gt;
&lt;servlet-name&gt;dispatcherServlet&lt;/servlet-name&gt;
&lt;servlet-class&gt;org.springframework.web.servlet.DispatcherServlet&lt;/servlet-class&gt;
&lt;/servlet&gt;</programlisting>
<para>The above filter intercepts HTTP PUT and PATCH requests with content type
<literal>application/x-www-form-urlencoded</literal>, reads the form
data from the body of the request, and wraps the
<classname>ServletRequest</classname> in order to make the form data
available through the
<literal>ServletRequest.getParameter*()</literal> family of
methods.</para>
<note>
<para>As <classname>HttpPutFormContentFilter</classname> consumes the body of the
request, it should not be configured for PUT or PATCH URLs that rely on other
converters for <literal>application/x-www-form-urlencoded</literal>. This includes
<literal>@RequestBody MultiValueMap&lt;String, String&gt;</literal> and
<literal>HttpEntity&lt;MultiValueMap&lt;String, String&gt;&gt;</literal>.</para>
</note>
</section>
<section xml:id="mvc-ann-cookievalue">
<title>Mapping cookie values with the @CookieValue annotation</title>
<para>The <interfacename>@CookieValue</interfacename> annotation
allows a method parameter to be bound to the value of an HTTP
cookie.</para>
<para>Let us consider that the following cookie has been received with
an http request:</para>
<programlisting>JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84</programlisting>
<para>The following code sample demonstrates how to get the value of
the <literal>JSESSIONID</literal> cookie:</para>
<programlisting language="java">@RequestMapping("/displayHeaderInfo.do")
public void displayHeaderInfo(<emphasis role="bold">@CookieValue("JSESSIONID")</emphasis> String cookie) {
//...
}</programlisting>
<para>Type conversion is applied automatically if the target method
parameter type is not <classname>String</classname>. See <xref
linkend="mvc-ann-typeconversion" />.</para>
<para>This annotation is supported for annotated handler methods in
Servlet and Portlet environments.</para>
</section>
<section xml:id="mvc-ann-requestheader">
<title>Mapping request header attributes with the @RequestHeader
annotation</title>
<para>The <interfacename>@RequestHeader</interfacename> annotation
allows a method parameter to be bound to a request header.</para>
<para>Here is a sample request header:</para>
<programlisting>
Host localhost:8080
Accept text/html,application/xhtml+xml,application/xml;q=0.9
Accept-Language fr,en-gb;q=0.7,en;q=0.3
Accept-Encoding gzip,deflate
Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive 300</programlisting>
<para>The following code sample demonstrates how to get the value of
the <literal>Accept-Encoding</literal> and
<literal>Keep-Alive</literal> headers:</para>
<programlisting language="java">@RequestMapping("/displayHeaderInfo.do")
public void displayHeaderInfo(<emphasis role="bold">@RequestHeader("Accept-Encoding")</emphasis> String encoding,
<emphasis role="bold">@RequestHeader("Keep-Alive")</emphasis> long keepAlive) {
//...
}</programlisting>
<para>Type conversion is applied automatically if the method parameter
is not <classname>String</classname>. See <xref
linkend="mvc-ann-typeconversion" />.</para>
<tip>
<para>Built-in support is available for converting a comma-separated
string into an array/collection of strings or other types known to
the type conversion system. For example a method parameter annotated
with <literal>@RequestHeader("Accept")</literal> may be of type
<classname>String</classname> but also
<classname>String[]</classname> or
<classname>List&lt;String&gt;</classname>.</para>
</tip>
<para>This annotation is supported for annotated handler methods in
Servlet and Portlet environments.</para>
</section>
<section xml:id="mvc-ann-typeconversion">
<title>Method Parameters And Type Conversion</title>
<para>String-based values extracted from the request including request
parameters, path variables, request headers, and cookie values may
need to be converted to the target type of the method parameter or
field (e.g., binding a request parameter to a field in an
<interfacename>@ModelAttribute</interfacename> parameter) they're
bound to. If the target type is not <classname>String</classname>,
Spring automatically converts to the appropriate type. All simple
types such as int, long, Date, etc. are supported. You can further
customize the conversion process through a
<classname>WebDataBinder</classname> (see <xref
linkend="mvc-ann-webdatabinder" />) or by registering
<classname>Formatters</classname> with the
<classname>FormattingConversionService</classname> (see <xref
linkend="format" />).</para>
</section>
<section xml:id="mvc-ann-webdatabinder">
<title>Customizing <classname>WebDataBinder</classname>
initialization</title>
<para>To customize request parameter binding with PropertyEditors
through Spring's <classname>WebDataBinder</classname>, you can use
<interfacename>@InitBinder</interfacename>-annotated methods within
your controller, <interfacename>@InitBinder</interfacename> methods
within an <interfacename>@ControllerAdvice</interfacename> class,
or provide a custom
<interfacename>WebBindingInitializer</interfacename>.</para>
<section xml:id="mvc-ann-initbinder">
<title>Customizing data binding with
<interfacename>@InitBinder</interfacename></title>
<para>Annotating controller methods with
<interfacename>@InitBinder</interfacename> allows you to configure
web data binding directly within your controller class.
<interfacename>@InitBinder</interfacename> identifies methods that
initialize the <classname>WebDataBinder</classname> that will be
used to populate command and form object arguments of annotated
handler methods.</para>
<para>Such init-binder methods support all arguments that
<interfacename>@RequestMapping</interfacename> supports, except for
command/form objects and corresponding validation result objects.
Init-binder methods must not have a return value. Thus, they are
usually declared as <literal>void</literal>. Typical arguments
include <classname>WebDataBinder</classname> in combination with
<interfacename>WebRequest</interfacename> or
<classname>java.util.Locale</classname>, allowing code to register
context-specific editors.</para>
<para>The following example demonstrates the use of
<interfacename>@InitBinder</interfacename> to configure a
<classname>CustomDateEditor</classname> for all
<classname>java.util.Date</classname> form properties.</para>
<programlisting language="java">@Controller
public class MyFormController {
<emphasis role="bold">@InitBinder</emphasis>
public void initBinder(WebDataBinder binder) {
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
dateFormat.setLenient(false);
binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
}
<lineannotation>// ...</lineannotation>
}</programlisting>
</section>
<section xml:id="mvc-ann-webbindinginitializer">
<title>Configuring a custom
<interfacename>WebBindingInitializer</interfacename></title>
<para>To externalize data binding initialization, you can provide a
custom implementation of the
<interfacename>WebBindingInitializer</interfacename> interface,
which you then enable by supplying a custom bean configuration for
an <classname>AnnotationMethodHandlerAdapter</classname>, thus
overriding the default configuration.</para>
<para>The following example from the PetClinic application shows a
configuration using a custom implementation of the
<interfacename>WebBindingInitializer</interfacename> interface,
<classname>org.springframework.samples.petclinic.web.ClinicBindingInitializer</classname>,
which configures PropertyEditors required by several of the
PetClinic controllers.</para>
<programlisting language="xml">&lt;bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"&gt;
&lt;property name="cacheSeconds" value="0" /&gt;
&lt;property name="webBindingInitializer"&gt;
&lt;bean class="org.springframework.samples.petclinic.web.ClinicBindingInitializer" /&gt;
&lt;/property&gt;
&lt;/bean&gt;</programlisting>
</section>
<section xml:id="mvc-ann-initbinder-advice">
<title>Customizing data binding with externalized
<interfacename>@InitBinder</interfacename> methods</title>
<para><interfacename>@InitBinder</interfacename> methods can also
be defined in an <interfacename>@ControllerAdvice</interfacename>-annotated
class in which case they apply to all controllers. This provides an
alternative to using a <interfacename>WebBindingInitializer</interfacename>.
</para>
<para>The <interfacename>@ControllerAdvice</interfacename> annotation is
a component annotation allowing implementation classes to be autodetected
through classpath scanning.</para>
</section>
</section>
<section xml:id="mvc-ann-lastmodified">
<title>Support for the 'Last-Modified' Response Header To Facilitate
Content Caching</title>
<para>An <interfacename>@RequestMapping</interfacename> method may
wish to support <literal>'Last-Modified'</literal> HTTP requests, as
defined in the contract for the Servlet API's
<literal>getLastModified</literal> method, to facilitate content
caching. This involves calculating a lastModified
<literal>long</literal> value for a given request, comparing it
against the <literal>'If-Modified-Since'</literal> request header
value, and potentially returning a response with status code 304 (Not
Modified). An annotated controller method can achieve that as
follows:</para>
<programlisting language="java">
@RequestMapping
public String myHandleMethod(WebRequest webRequest, Model model) {
long lastModified = // 1. application-specific calculation
if (request.checkNotModified(lastModified)) {
// 2. shortcut exit - no further processing necessary
return null;
}
// 3. or otherwise further request processing, actually preparing content
model.addAttribute(...);
return "myViewName";
}</programlisting>
<para>There are two key elements to note: calling
<code>request.checkNotModified(lastModified)</code> and returning
<literal>null</literal>. The former sets the response status to 304
before it returns <literal>true</literal>. The latter, in combination
with the former, causes Spring MVC to do no further processing of the
request.</para>
</section>
</section>
<section xml:id="mvc-ann-async">
<title>Asynchronous Request Processing</title>
<para>Spring MVC 3.2 introduced Servlet 3 based asynchronous request
processing. Instead of returning a value, as usual, a controller method
can now return a <interfacename>java.util.concurrent.Callable</interfacename>
and produce the return value from a separate thread. Meanwhile the main Servlet
container thread is released and allowed to process other requests.
Spring MVC invokes the <interfacename>Callable</interfacename> in a
separate thread with the help of a <interfacename>TaskExecutor</interfacename>
and when the <interfacename>Callable</interfacename> returns, the
request is dispatched back to the Servlet container to resume
processing with the value returned by the
<interfacename>Callable</interfacename>.
Here is an example controller method:</para>
<programlisting language="java">
@RequestMapping(method=RequestMethod.POST)
public Callable&lt;String&gt; processUpload(final MultipartFile file) {
return new Callable&lt;String&gt;() {
public String call() throws Exception {
// ...
return "someView";
}
};
}</programlisting>
<para>A second option is for the controller to return an instance of
<classname>DeferredResult</classname>. In this case the return value
will also be produced from a separate thread. However, that thread is not
known to Spring MVC. For example the result may be produced in response
to some external event such as a JMS message, a scheduled task, etc.
Here is an example controller method:</para>
<programlisting language="java">
@RequestMapping("/quotes")
@ResponseBody
public DeferredResult&lt;String&gt; quotes() {
DeferredResult&lt;String&gt; deferredResult = new DeferredResult&lt;String&gt;();
// Save the deferredResult in in-memory queue ...
return deferredResult;
}
// In some other thread...
deferredResult.setResult(data);
</programlisting>
<para>This may be difficult to understand without any knowledge of the
Servlet 3 async processing feature. It would certainly help to read up on it.
At a very minimum consider the following basic facts:</para>
<itemizedlist>
<listitem>
<para>A <interfacename>ServletRequest</interfacename>
can be put in asynchronous mode by calling
<code>request.startAsync()</code>. The main effect of doing so is
that the Servlet, as well as any Filters, can exit but the response
will remain open allowing some other thread to complete processing.
</para>
</listitem>
<listitem>
<para>The call to <code>request.startAsync()</code> returns an
<interfacename>AsyncContext</interfacename>, which can be used for
further control over async processing. For example it provides
the method <code>dispatch</code>, which can be called from an
application thread in order to "dispatch" the request back to
the Servlet container. An async dispatch is similar to a forward
except it is made from one (application) thread to another
(Servlet container) thread whereas a forward occurs synchronously
in the same (Servlet container) thread.</para>
</listitem>
<listitem>
<para><interfacename>ServletRequest</interfacename> provides access
to the current <interfacename>DispatcherType</interfacename>, which
can be used to distinguish if a <interfacename>Servlet</interfacename> or
a <interfacename>Filter</interfacename> is processing on
the initial request processing thread and when it is processing in
an async dispatch.</para>
</listitem>
</itemizedlist>
<para>With the above in mind, the following is the sequence
of events for async request processing with a <interfacename>Callable</interfacename>:
(1) Controller returns a
<interfacename>Callable</interfacename>, (2) Spring MVC starts async processing
and submits the <interfacename>Callable</interfacename>
to a <interfacename>TaskExecutor</interfacename>
for processing in a separate thread, (3) the <classname>DispatcherServlet</classname>
and all Filter's exit the request processing thread but the response
remains open, (4) the <interfacename>Callable</interfacename> produces a result
and Spring MVC dispatches the request back to the Servlet container,
(5) the <classname>DispatcherServlet</classname> is invoked again and processing
resumes with the asynchronously produced result from the
<interfacename>Callable</interfacename>. The exact sequencing of (2),
(3), and (4) may vary depending on the speed of execution of the
concurrent threads.
</para>
<para>The sequence of events for async request processing with a
<classname>DeferredResult</classname> is the same in principal except
it's up to the application to produce the asynchronous result from some thread:
(1) Controller returns a <classname>DeferredResult</classname> and saves it
in some in-memory queue or list where it can be accessed,
(2) Spring MVC starts async processing, (3) the <classname>DispatcherServlet</classname>
and all configured Filter's exit the request processing thread but the response
remains open, (4) the application sets the <classname>DeferredResult</classname>
from some thread and Spring MVC dispatches the request back to the Servlet container,
(5) the <classname>DispatcherServlet</classname> is invoked again and processing
resumes with the asynchronously produced result.
</para>
<para>Explaining the motivation for async request processing and when or why to use it
are beyond the scope of this document. For further information you may wish to read
<link xl:href="http://blog.springsource.org/2012/05/06/spring-mvc-3-2-preview-introducing-servlet-3-async-support/">this blog post series</link>.
</para>
<section xml:id="mvc-ann-async-exceptions">
<title>Exception Handling for Async Requests</title>
<para>What happens if a <interfacename>Callable</interfacename> returned
from a controller method raises an Exception while being executed?
The effect is similar to what happens when any controller method raises
an exception. It is handled by a matching
<interfacename>@ExceptionHandler</interfacename> method in the same
controller or by one of the configured
<interfacename>HandlerExceptionResolver</interfacename> instances.</para>
<note>
<para>Under the covers, when a <interfacename>Callable</interfacename>
raises an Exception, Spring MVC still dispatches to the Servlet
container to resume processing. The only difference is that the
result of executing the <interfacename>Callable</interfacename>
is an <classname>Exception</classname> that must be processed
with the configured
<interfacename>HandlerExceptionResolver</interfacename> instances.</para>
</note>
<para>When using a <classname>DeferredResult</classname>, you have
a choice of calling its <code>setErrorResult(Object)</code> method
and provide an <classname>Exception</classname> or any other Object
you'd like to use as the result. If the result is an
<classname>Exception</classname>, it will be processed with a
matching <interfacename>@ExceptionHandler</interfacename> method in the
same controller or with any configured
<interfacename>HandlerExceptionResolver</interfacename> instance.</para>
</section>
<section xml:id="mvc-ann-async-interception">
<title>Intercepting Async Requests</title>
<para>An existing <interfacename>HandlerInterceptor</interfacename> can
implement <interfacename>AsyncHandlerInterceptor</interfacename>, which
provides one additional method <code>afterConcurrentHandlingStarted</code>.
It is invoked after async processing starts and when the initial
request processing thread is being exited. See the Javadoc of
<interfacename>AsyncHandlerInterceptor</interfacename> for more details
on that.</para>
<para>Further options for async request lifecycle callbacks are
provided directly on <classname>DeferredResult</classname>,
which has the methods <code>onTimeout(Runnable)</code> and
<code>onCompletion(Runnable)</code>. Those are called when the
async request is about to time out or has completed respectively.
The timeout event can be handled by setting the
<classname>DeferredResult</classname> to some value.
The completion callback however is final and the result can no
longer be set.</para>
<para>Similar callbacks are also available with a
<interfacename>Callable</interfacename>. However, you will need to wrap
the <interfacename>Callable</interfacename> in an instance of
<classname>WebAsyncTask</classname> and then use that to register
the timeout and completion callbacks. Just like with
<classname>DeferredResult</classname>, the timeout event can be
handled and a value can be returned while the completion event is final.</para>
<para>You can also register a
<interfacename>CallableProcessingInterceptor</interfacename> or a
<interfacename>DeferredResultProcessingInterceptor</interfacename>
globally through the MVC Java config or the MVC namespace.
Those interceptors provide a full set of callbacks and apply every
time a <interfacename>Callable</interfacename> or a
<classname>DeferredResult</classname> is used.</para>
</section>
<section xml:id="mvc-ann-async-configuration">
<title>Configuration for Async Request Processing</title>
<section xml:id="mvc-ann-async-configuration-servlet3">
<title>Servlet 3 Async Config</title>
<para>To use Servlet 3 async request processing, you need to update
<filename>web.xml</filename> to version 3.0:</para>
<programlisting language="xml">&lt;web-app xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
version="3.0"&gt;
...
&lt;/web-app&gt;
</programlisting>
<para>The <classname>DispatcherServlet</classname> and any
<interfacename>Filter</interfacename> configuration need to have
the <code>&lt;async-supported&gt;true&lt;/async-supported&gt;</code> sub-element.
Additionally, any <interfacename>Filter</interfacename> that also needs
to get involved in async dispatches should also be configured
to support the ASYNC dispatcher type. Note that it is safe
to enable the ASYNC dispatcher type for all filters provided with
the Spring Framework since they will not get involved in async
dispatches unless needed.</para>
<para>If using Servlet 3, Java based configuration, e.g. via
<interfacename>WebApplicationInitializer</interfacename>, you'll
also need to set the "asyncSupported" flag as well as the
ASYNC dispatcher type just like with <filename>web.xml</filename>.
To simplify all this configuration, consider
extending <classname>AbstractDispatcherServletInitializer</classname>
or <classname>AbstractAnnotationConfigDispatcherServletInitializer</classname>,
which automatically set those options and make it very easy to register
<interfacename>Filter</interfacename> instances.</para>
</section>
<section xml:id="mvc-ann-async-configuration-spring-mvc">
<title>Spring MVC Async Config</title>
<para>The MVC Java config and the MVC namespace both provide options for
configuring async request processing.
<interfacename>WebMvcConfigurer</interfacename> has the method
<code>configureAsyncSupport</code> while &lt;mvc:annotation-driven&gt;
has an &lt;async-support&gt; sub-element.</para>
<para>Those allow you to configure the default timeout value to use for
async requests, which if not set depends on the underlying Servlet
container (e.g. 10 seconds on Tomcat). You can also configure an
<interfacename>AsyncTaskExecutor</interfacename> to use for executing
<interfacename>Callable</interfacename> instances returned from
controller methods. It is highly recommended to configure this property
since by default Spring MVC uses
<classname>SimpleAsyncTaskExecutor</classname>. The MVC Java config
and the MVC namespace also allow you to register
<interfacename>CallableProcessingInterceptor</interfacename> and
<interfacename>DeferredResultProcessingInterceptor</interfacename>
instances.</para>
<para>If you need to override the default timeout value for a
specific <classname>DeferredResult</classname>, you can do so by using
the appropriate class constructor. Similarly, for a
<interfacename>Callable</interfacename>, you can wrap it in a
<classname>WebAsyncTask</classname> and use the appropriate class
constructor to customize the timeout value. The class constructor of
<classname>WebAsyncTask</classname> also allows providing
an <interfacename>AsyncTaskExecutor</interfacename>.</para>
</section>
</section>
</section>
<section xml:id="mvc-ann-tests">
<title>Testing Controllers</title>
<para>The <filename>spring-test</filename> module offers first class support
for testing annotated controllers.
See <xref linkend="spring-mvc-test-framework"/>.</para>
</section>
</section>
<section xml:id="mvc-handlermapping">
<title>Handler mappings</title>
<para>In previous versions of Spring, users were required to define one or
more <interfacename>HandlerMapping</interfacename> beans in the web
application context to map incoming web requests to appropriate handlers.
With the introduction of annotated controllers, you generally don't need
to do that because the <classname>RequestMappingHandlerMapping</classname>
automatically looks for <interfacename>@RequestMapping</interfacename>
annotations on all <interfacename>@Controller</interfacename> beans.
However, do keep in mind that all <classname>HandlerMapping</classname>
classes extending from <classname>AbstractHandlerMapping</classname> have
the following properties that you can use to customize their
behavior:</para>
<variablelist>
<varlistentry>
<term><literal>interceptors</literal></term>
<listitem>
<para>List of interceptors to use.
<interfacename>HandlerInterceptor</interfacename>s are discussed in
<xref linkend="mvc-handlermapping-interceptor" />.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>defaultHandler</literal></term>
<listitem>
<para>Default handler to use, when this handler mapping does not
result in a matching handler.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>order</literal></term>
<listitem>
<para>Based on the value of the order property (see the
<literal>org.springframework.core.Ordered</literal> interface),
Spring sorts all handler mappings available in the context and
applies the first matching handler.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>alwaysUseFullPath</literal></term>
<listitem>
<para>If <literal>true</literal> , Spring uses the full path within
the current Servlet context to find an appropriate handler. If
<literal>false</literal> (the default), the path within the current
Servlet mapping is used. For example, if a Servlet is mapped using
<literal>/testing/*</literal> and the
<literal>alwaysUseFullPath</literal> property is set to true,
<literal>/testing/viewPage.html</literal> is used, whereas if the
property is set to false, <literal>/viewPage.html</literal> is
used.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>urlDecode</literal></term>
<listitem>
<para>Defaults to <literal>true</literal>, as of Spring 2.5. If you
prefer to compare encoded paths, set this flag to
<literal>false</literal>. However, the
<interfacename>HttpServletRequest</interfacename> always exposes the
Servlet path in decoded form. Be aware that the Servlet path will
not match when compared with encoded paths.</para>
</listitem>
</varlistentry>
</variablelist>
<para>The following example shows how to configure an interceptor:</para>
<programlisting language="xml">&lt;beans&gt;
&lt;bean id="handlerMapping" class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"&gt;
&lt;property name="interceptors"&gt;
&lt;bean class="example.MyInterceptor"/&gt;
&lt;/property&gt;
&lt;/bean&gt;
&lt;beans&gt;</programlisting>
<section xml:id="mvc-handlermapping-interceptor">
<title>Intercepting requests with a
<interfacename>HandlerInterceptor</interfacename></title>
<para>Spring's handler mapping mechanism includes handler interceptors,
which are useful when you want to apply specific functionality to
certain requests, for example, checking for a principal.</para>
<para>Interceptors located in the handler mapping must implement
<interfacename>HandlerInterceptor</interfacename> from the
<literal>org.springframework.web.servlet</literal> package. This
interface defines three methods: <literal>preHandle(..)</literal> is
called <emphasis>before</emphasis> the actual handler is executed;
<literal>postHandle(..)</literal> is called <emphasis>after</emphasis>
the handler is executed; and <literal>afterCompletion(..)</literal> is
called <emphasis>after the complete request has finished</emphasis>.
These three methods should provide enough flexibility to do all kinds of
preprocessing and postprocessing.</para>
<para>The <literal>preHandle(..)</literal> method returns a boolean
value. You can use this method to break or continue the processing of
the execution chain. When this method returns <literal>true</literal>,
the handler execution chain will continue; when it returns false, the
<classname>DispatcherServlet</classname> assumes the interceptor itself
has taken care of requests (and, for example, rendered an appropriate
view) and does not continue executing the other interceptors and the
actual handler in the execution chain.</para>
<para>Interceptors can be configured using the
<literal>interceptors</literal> property, which is present on all
<classname>HandlerMapping</classname> classes extending from
<classname>AbstractHandlerMapping</classname>. This is shown in the
example below:</para>
<programlisting language="xml">&lt;beans&gt;
&lt;bean id="handlerMapping"
class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"&gt;
&lt;property name="interceptors"&gt;
&lt;list&gt;
&lt;ref bean="officeHoursInterceptor"/&gt;
&lt;/list&gt;
&lt;/property&gt;
&lt;/bean&gt;
&lt;bean id="officeHoursInterceptor"
class="samples.TimeBasedAccessInterceptor"&gt;
&lt;property name="openingTime" value="9"/&gt;
&lt;property name="closingTime" value="18"/&gt;
&lt;/bean&gt;
&lt;beans&gt;</programlisting>
<programlisting language="java">package samples;
public class TimeBasedAccessInterceptor extends HandlerInterceptorAdapter {
private int openingTime;
private int closingTime;
public void setOpeningTime(int openingTime) {
this.openingTime = openingTime;
}
public void setClosingTime(int closingTime) {
this.closingTime = closingTime;
}
public boolean preHandle(
HttpServletRequest request,
HttpServletResponse response,
Object handler) throws Exception {
Calendar cal = Calendar.getInstance();
int hour = cal.get(HOUR_OF_DAY);
if (openingTime &lt;= hour &amp;&amp; hour &lt; closingTime) {
return true;
} else {
response.sendRedirect("http://host.com/outsideOfficeHours.html");
return false;
}
}
}</programlisting>
<para>Any request handled by this mapping is intercepted by the
<classname>TimeBasedAccessInterceptor</classname>. If the current time
is outside office hours, the user is redirected to a static HTML file
that says, for example, you can only access the website during office
hours.</para>
<note>
<para>When using the
<classname>RequestMappingHandlerMapping</classname> the actual handler
is an instance of <classname>HandlerMethod</classname> which
identifies the specific controller method that will be invoked.</para>
</note>
<para>As you can see, the Spring adapter class
<classname>HandlerInterceptorAdapter</classname> makes it easier to
extend the <interfacename>HandlerInterceptor</interfacename>
interface.</para>
<tip>
<para>In the example above, the configured interceptor will apply to
all requests handled with annotated controller methods. If you want to
narrow down the URL paths to which an interceptor applies, you can use
the MVC namespace or the MVC Java config, or declare bean instances
of type <classname>MappedInterceptor</classname> to do that. See <link
linkend="mvc-config-enable">Enabling the MVC Java Config or the MVC
XML Namespace</link>.</para>
</tip>
</section>
</section>
<section xml:id="mvc-viewresolver">
<title>Resolving views</title>
<para>All MVC frameworks for web applications provide a way to address
views. Spring provides view resolvers, which enable you to render models
in a browser without tying you to a specific view technology. Out of the
box, Spring enables you to use JSPs, Velocity templates and XSLT views,
for example. See <xref linkend="view" /> for a discussion of how to
integrate and use a number of disparate view technologies.</para>
<para>The two interfaces that are important to the way Spring handles
views are <interfacename>ViewResolver</interfacename> and
<interfacename>View</interfacename>. The
<interfacename>ViewResolver</interfacename> provides a mapping between
view names and actual views. The <interfacename>View</interfacename>
interface addresses the preparation of the request and hands the request
over to one of the view technologies.</para>
<section xml:id="mvc-viewresolver-resolver">
<title>Resolving views with the
<interfacename>ViewResolver</interfacename> interface</title>
<para>As discussed in <xref linkend="mvc-controller" />, all handler
methods in the Spring Web MVC controllers must resolve to a logical view
name, either explicitly (e.g., by returning a <literal>String</literal>,
<literal>View</literal>, or <literal>ModelAndView</literal>) or
implicitly (i.e., based on conventions). Views in Spring are addressed
by a logical view name and are resolved by a view resolver. Spring comes
with quite a few view resolvers. This table lists most of them; a couple
of examples follow.</para>
<table xml:id="mvc-view-resolvers-tbl">
<title>View resolvers</title>
<tgroup cols="2">
<colspec colname="c1" colwidth="1*" />
<colspec colname="c2" colwidth="2*" />
<thead>
<row>
<entry><interfacename>ViewResolver</interfacename></entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><classname>AbstractCachingViewResolver</classname></entry>
<entry>Abstract view resolver that caches views. Often views
need preparation before they can be used; extending this view
resolver provides caching.</entry>
</row>
<row>
<entry><classname>XmlViewResolver</classname></entry>
<entry>Implementation of
<interfacename>ViewResolver</interfacename> that accepts a
configuration file written in XML with the same DTD as Spring's
XML bean factories. The default configuration file is
<literal>/WEB-INF/views.xml</literal>.</entry>
</row>
<row>
<entry><classname>ResourceBundleViewResolver</classname></entry>
<entry>Implementation of
<interfacename>ViewResolver</interfacename> that uses bean
definitions in a <classname>ResourceBundle</classname>,
specified by the bundle base name. Typically you define the
bundle in a properties file, located in the classpath. <!--Correct to say you define? Seems so, because default implies you can change it.-->The
default file name is
<literal>views.properties</literal>.</entry>
</row>
<row>
<entry><classname>UrlBasedViewResolver</classname></entry>
<entry>Simple implementation of the
<interfacename>ViewResolver</interfacename> interface that
effects the direct resolution of logical view names to URLs,
without an explicit mapping definition. This is appropriate if
your logical names match the names of your view resources in a
straightforward manner, without the need for arbitrary
mappings.</entry>
</row>
<row>
<entry><classname>InternalResourceViewResolver</classname></entry>
<entry>Convenient subclass of
<classname>UrlBasedViewResolver</classname> that supports
<classname>InternalResourceView</classname> (in effect, Servlets
and JSPs) and subclasses such as <classname>JstlView</classname>
and <classname>TilesView</classname>. You can specify the view
class for all views generated by this resolver by using
<literal>setViewClass(..)</literal>. See the Javadocs for the
<classname>UrlBasedViewResolver</classname> class for
details.</entry>
</row>
<row>
<entry><classname>VelocityViewResolver</classname> /
<classname>FreeMarkerViewResolver</classname></entry>
<entry>Convenient subclass of
<classname>UrlBasedViewResolver</classname> that supports
<classname>VelocityView</classname> (in effect, Velocity
templates) or <classname>FreeMarkerView</classname>
,respectively, and custom subclasses of them.</entry>
</row>
<row>
<entry><classname>ContentNegotiatingViewResolver</classname></entry>
<entry>Implementation of the
<interfacename>ViewResolver</interfacename> interface that
resolves a view based on the request file name or
<literal>Accept</literal> header. See <xref
linkend="mvc-multiple-representations" />.</entry>
</row>
</tbody>
</tgroup>
</table>
<para>As an example, with JSP as a view technology, you can use the
<classname>UrlBasedViewResolver</classname>. This view resolver
translates a view name to a URL and hands the request over to the
RequestDispatcher to render the view.</para>
<programlisting language="xml">&lt;bean id="viewResolver"
class="org.springframework.web.servlet.view.UrlBasedViewResolver"&gt;
&lt;property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/&gt;
&lt;property name="prefix" value="/WEB-INF/jsp/"/&gt;
&lt;property name="suffix" value=".jsp"/&gt;
&lt;/bean&gt;</programlisting>
<para>When returning <literal>test</literal> as a logical view name,
this view resolver forwards the request to the
<classname>RequestDispatcher</classname> that will send the request to
<literal>/WEB-INF/jsp/test.jsp</literal>.</para>
<para>When you combine different view technologies in a web application,
you can use the
<classname>ResourceBundleViewResolver</classname>:</para>
<programlisting language="xml">&lt;bean id="viewResolver"
class="org.springframework.web.servlet.view.ResourceBundleViewResolver"&gt;
&lt;property name="basename" value="views"/&gt;
&lt;property name="defaultParentView" value="parentView"/&gt;
&lt;/bean&gt;</programlisting>
<para>The <classname>ResourceBundleViewResolver</classname> inspects the
<classname>ResourceBundle</classname> identified by the basename, and
for each view it is supposed to resolve, it uses the value of the
property <literal>[viewname].(class)</literal> as the view class and the
value of the property <literal>[viewname].url</literal> as the view url.
Examples can be found in the next chapter which covers view
technologies. As you can see, you can identify a parent view, from which
all views in the properties file <quote>extend</quote>. This way you can
specify a default view class, for example.</para>
<note>
<para>Subclasses of <classname>AbstractCachingViewResolver</classname>
cache view instances that they resolve. Caching improves performance
of certain view technologies. It's possible to turn off the cache by
setting the <literal>cache</literal> property to
<literal>false</literal>. Furthermore, if you must refresh a certain
view at runtime (for example when a Velocity template is modified),
you can use the <literal>removeFromCache(String viewName, Locale
loc)</literal> method.</para>
</note>
</section>
<section xml:id="mvc-viewresolver-chaining">
<title>Chaining ViewResolvers</title>
<para>Spring supports multiple view resolvers. Thus you can chain
resolvers and, for example, override specific views in certain
circumstances. You chain view resolvers by adding more than one resolver
to your application context and, if necessary, by setting the
<literal>order</literal> property to specify ordering. Remember, the
higher the order property, the later the view resolver is positioned in
the chain.</para>
<para>In the following example, the chain of view resolvers consists of
two resolvers, an <classname>InternalResourceViewResolver</classname>,
which is always automatically positioned as the last resolver in the
chain, and an <classname>XmlViewResolver</classname> for specifying
Excel views. Excel views are not supported by the
<classname>InternalResourceViewResolver</classname>.<!--Do you need to say anything else about excel not being supported by one of resolvers? What if anything is the result?--></para>
<programlisting language="xml">&lt;bean id="jspViewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver"&gt;
&lt;property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/&gt;
&lt;property name="prefix" value="/WEB-INF/jsp/"/&gt;
&lt;property name="suffix" value=".jsp"/&gt;
&lt;/bean&gt;
&lt;bean id="excelViewResolver" class="org.springframework.web.servlet.view.XmlViewResolver"&gt;
&lt;property name="order" value="1"/&gt;
&lt;property name="location" value="/WEB-INF/views.xml"/&gt;
&lt;/bean&gt;
<lineannotation>&lt;!-- in views.xml --&gt;</lineannotation>
&lt;beans&gt;
&lt;bean name="report" class="org.springframework.example.ReportExcelView"/&gt;
&lt;/beans&gt;</programlisting>
<para>If a specific view resolver does not result in a view, Spring
examines the context for other view resolvers. If additional view
resolvers exist, Spring continues to inspect them until a view is
resolved. If no view resolver returns a view, Spring throws a
<classname>ServletException</classname>.</para>
<para>The contract of a view resolver specifies that a view resolver
<emphasis>can</emphasis> return null to indicate the view could not be
found. Not all view resolvers do this, however, because in some cases,
the resolver simply cannot detect whether or not the view exists. For
example, the <classname>InternalResourceViewResolver</classname> uses
the <classname>RequestDispatcher</classname> internally, and dispatching
is the only way to figure out if a JSP exists, but this action can only
execute once. The same holds for the
<classname>VelocityViewResolver</classname> and some others. Check the
Javadoc for the view resolver to see whether it reports non-existing
views. Thus, putting an
<classname>InternalResourceViewResolver</classname> in the chain in a
place other than the last, results in the chain not being fully
inspected, because the
<classname>InternalResourceViewResolver</classname> will
<emphasis>always</emphasis> return a view!<!--I don't understand the logic of this. How can it return a view if no view exists or no view can be found? this paragraph is confusing.--><!--Why would you put InternalResourceViewResolver in place other than last? It's automatically last. --></para>
</section>
<section xml:id="mvc-redirecting">
<title>Redirecting to views<!--Revise to say what you are redirecting to views. OR are you redirecting views? In that case heading should be Redirecting views.--></title>
<para>As mentioned previously, a controller typically returns a logical
view name, which a view resolver resolves to a particular view
technology. For view technologies such as JSPs that are processed
through the Servlet or JSP engine, this resolution is usually handled
through the combination of
<classname>InternalResourceViewResolver</classname> and
<classname>InternalResourceView</classname>, which issues an internal
forward or include via the Servlet API's
<literal>RequestDispatcher.forward(..)</literal> method or
<literal>RequestDispatcher.include()</literal> method. For other view
technologies, such as Velocity, XSLT, and so on, the view itself writes
the content directly to the response stream.</para>
<para>It is sometimes desirable to issue an HTTP redirect back to the
client, before the view is rendered. This is desirable, for example,
when one controller has been called with <literal>POST</literal>ed data,
and the response is actually a delegation to another controller (for
example on a successful form submission). In this case, a normal
internal forward will mean that the other controller will also see the
same <literal>POST</literal> data, which is potentially problematic if
it can confuse it with other expected data. Another reason to perform a
redirect before displaying the result is to eliminate the possibility of
the user submitting the form data multiple times. In this scenario, the
browser will first send an initial <literal>POST</literal>; it will then
receive a response to redirect to a different URL; and finally the
browser will perform a subsequent <literal>GET</literal> for the URL
named in the redirect response. Thus, from the perspective of the
browser, the current page does not reflect the result of a
<literal>POST</literal> but rather of a <literal>GET</literal>. The end
effect is that there is no way the user can accidentally
re-<literal>POST</literal> the same data by performing a refresh. The
refresh forces a <literal>GET</literal> of the result page, not a resend
of the initial <literal>POST</literal> data.</para>
<section xml:id="mvc-redirecting-redirect-view">
<title><classname>RedirectView</classname></title>
<para>One way to force a redirect as the result of a controller
response is for the controller to create and return an instance of
Spring's <classname>RedirectView</classname>. In this case,
<classname>DispatcherServlet</classname> does not use the normal view
resolution mechanism. Rather because it has been given the (redirect)
view already, the <classname>DispatcherServlet</classname> simply
instructs the view to do its work.</para>
<para>The <classname>RedirectView</classname> issues an
<literal>HttpServletResponse.sendRedirect()</literal> call that
returns to the client browser as an HTTP redirect. By default all
model attributes are considered to be exposed as URI template
variables in the redirect URL. Of the remaining attributes those that
are primitive types or collections/arrays of primitive types are
automatically appended as query parameters.</para>
<para>Appending primitive type attributes as query parameters may be
the desired result if a model instance was prepared specifically for
the redirect. However, in annotated controllers the model may contain
additional attributes added for rendering purposes (e.g. drop-down
field values). To avoid the possibility of having such attributes
appear in the URL an annotated controller can declare an argument of
type <interfacename>RedirectAttributes</interfacename> and use it to
specify the exact attributes to make available to
<classname>RedirectView</classname>. If the controller method decides
to redirect, the content of
<interfacename>RedirectAttributes</interfacename> is used. Otherwise
the content of the model is used.</para>
<para>Note that URI template variables from the present request are
automatically made available when expanding a redirect URL and do not
need to be added explicitly neither through
<interfacename>Model</interfacename> nor
<interfacename>RedirectAttributes</interfacename>. For example:</para>
<programlisting language="java">@RequestMapping(value = "/files/{path}", method = RequestMethod.POST)
public String upload(...) {
// ...
return "redirect:files/{path}";
}</programlisting>
<para>If you use <classname>RedirectView</classname> and the view is
created by the controller itself, it is recommended that you configure
the redirect URL to be injected into the controller so that it is not
baked into the controller but configured in the context along with the
view names. The next section discusses this process.</para>
</section>
<section xml:id="mvc-redirecting-redirect-prefix">
<title>The <literal>redirect:</literal> prefix</title>
<para>While the use of <classname>RedirectView</classname> works fine,
if the controller itself creates the
<classname>RedirectView</classname>, there is no avoiding the fact
that the controller is aware that a redirection is happening. This is
really suboptimal and couples things too tightly. The controller
should not really care about how the response gets handled. In general
it should operate only in terms of view names that have been injected
into it.</para>
<para>The special <literal>redirect:</literal> prefix allows you to
accomplish this. If a view name is returned that has the prefix
<literal>redirect:</literal>, the
<classname>UrlBasedViewResolver</classname> (and all subclasses) will
recognize this as a special indication that a redirect is needed. The
rest of the view name will be treated as the redirect URL.</para>
<para>The net effect is the same as if the controller had returned a
<classname>RedirectView</classname>, but now the controller itself can
simply operate in terms of logical view names. A logical view name
such as <literal>redirect:/myapp/some/resource</literal> will redirect
relative to the current Servlet context, while a name such as
<literal>redirect:http://myhost.com/some/arbitrary/path</literal> will
redirect to an absolute URL.</para>
</section>
<section xml:id="mvc-redirecting-forward-prefix">
<title>The <literal>forward:</literal> prefix<!--Can you revise this heading to say what you're using the forward prefix to accomplish?--></title>
<para>It is also possible to use a special <literal>forward:</literal>
prefix for view names that are ultimately resolved by
<classname>UrlBasedViewResolver</classname> and subclasses. This
creates an <classname>InternalResourceView</classname> (which
ultimately does a <literal>RequestDispatcher.forward()</literal>)
around the rest of the view name, which is considered a URL.
Therefore, this prefix is not useful with
<classname>InternalResourceViewResolver</classname> and
<classname>InternalResourceView</classname> (for JSPs for example).
But the prefix can be helpful when you are primarily using another
view technology, but still want to force a forward of a resource to be
handled by the Servlet/JSP engine. (Note that you may also chain
multiple view resolvers, instead.)<!--I think the preceding sentences were a bit garbled. I tried to reword a bit. And is this paragraph logical?--></para>
<para>As with the <literal>redirect:</literal> prefix, if the view
name with the <literal>forward:</literal> prefix is injected into the
controller, the controller does not detect that anything special is
happening in terms of handling the response.<!--Can you reword to clarify the point? The controller does not detect what?--></para>
</section>
</section>
<section xml:id="mvc-multiple-representations">
<title><classname>ContentNegotiatingViewResolver</classname></title>
<para>The <classname>ContentNegotiatingViewResolver</classname> does not
resolve views itself but rather delegates to other view resolvers,
selecting the view that resembles the representation requested by the
client. Two strategies exist for a client to request a representation
from the server:</para>
<itemizedlist>
<listitem>
<para>Use a distinct URI for each resource, typically by using a
different file extension in the URI. For example, the URI<literal>
http://www.example.com/users/fred.pdf</literal> requests a PDF
representation of the user fred, and
<literal>http://www.example.com/users/fred.xml</literal> requests an
XML representation.</para>
</listitem>
</itemizedlist>
<itemizedlist>
<listitem>
<para>Use the same URI for the client to locate the resource, but
set the <literal>Accept</literal> HTTP request header to list the
<link xl:href="http://en.wikipedia.org/wiki/Internet_media_type">media
types</link> that it understands. For example, an HTTP request for
<literal>http://www.example.com/users/fred</literal> with an
<literal>Accept</literal> header set to <literal>application/pdf
</literal>requests a PDF representation of the user fred, while
<literal>http://www.example.com/users/fred</literal> with an
<literal>Accept</literal> header set to <literal>text/xml</literal>
requests an XML representation. This strategy is known as <link
xl:href="http://en.wikipedia.org/wiki/Content_negotiation">content
negotiation</link>.</para>
</listitem>
</itemizedlist>
<note>
<para>One issue with the <literal>Accept</literal> header is that it
is impossible to set it in a web browser within HTML. For example, in
Firefox, it is fixed to:<!--So how would you set the Accept header as in second bullet, if you can't do it in html? Indicate?--></para>
<programlisting>Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8</programlisting>
<para>For this reason it is common to see the use of a distinct URI
for each representation when developing browser based web
applications.</para>
</note>
<para>To support multiple representations of a resource, Spring provides
the <classname>ContentNegotiatingViewResolver</classname> to resolve a
view based on the file extension or <literal>Accept</literal> header of
the HTTP request. <classname>ContentNegotiatingViewResolver</classname>
does not perform the view resolution itself but instead delegates to a
list of view resolvers that you specify through the bean property
<literal>ViewResolvers</literal>.<!--A human has to specify this list of resolvers, right? See example below.--></para>
<para>The <classname>ContentNegotiatingViewResolver</classname> selects
an appropriate <classname>View</classname> to handle the request by
comparing the request media type(s) with the media type (also known as
<literal>Content-Type</literal>) supported by the
<classname>View</classname> associated with each of its
<classname>ViewResolvers</classname>. The first
<classname>View</classname> in the list that has a compatible
<literal>Content-Type</literal> returns the representation to the
client. If a compatible view cannot be supplied by the
<classname>ViewResolver</classname> chain, then the list of views
specified through the <literal>DefaultViews</literal> property will be
consulted. This latter option is appropriate for singleton
<classname>Views</classname> that can render an appropriate
representation of the current resource regardless of the logical view
name. The <literal>Accept</literal> header may include wild cards, for
example <literal>text/*</literal>, in which case a
<classname>View</classname> whose Content-Type was
<literal>text/xml</literal> is a compatible match.</para>
<para>To support the resolution of a view based on a file extension, use
the <classname>ContentNegotiatingViewResolver </classname>bean property
<literal>mediaTypes</literal> to specify a mapping of file extensions to
media types. For more information on the algorithm used to determine the
request media type, refer to the API documentation for
<classname>ContentNegotiatingViewResolver</classname>.</para>
<para>Here is an example configuration of a
<classname>ContentNegotiatingViewResolver:</classname></para>
<programlisting language="xml">&lt;bean class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver"&gt;
&lt;property name="mediaTypes"&gt;
&lt;map&gt;
&lt;entry key="atom" value="application/atom+xml"/&gt;
&lt;entry key="html" value="text/html"/&gt;
&lt;entry key="json" value="application/json"/&gt;
&lt;/map&gt;
&lt;/property&gt;
&lt;property name="viewResolvers"&gt;
&lt;list&gt;
&lt;bean class="org.springframework.web.servlet.view.BeanNameViewResolver"/&gt;
&lt;bean class="org.springframework.web.servlet.view.InternalResourceViewResolver"&gt;
&lt;property name="prefix" value="/WEB-INF/jsp/"/&gt;
&lt;property name="suffix" value=".jsp"/&gt;
&lt;/bean&gt;
&lt;/list&gt;
&lt;/property&gt;
&lt;property name="defaultViews"&gt;
&lt;list&gt;
&lt;bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView" /&gt;
&lt;/list&gt;
&lt;/property&gt;
&lt;/bean&gt;
&lt;bean id="content" class="com.springsource.samples.rest.SampleContentAtomView"/&gt;</programlisting>
<para>The <classname>InternalResourceViewResolver</classname> handles
the translation of view names and JSP pages, while the
<classname>BeanNameViewResolver</classname> returns a view based on the
name of a bean. (See "<link
linkend="mvc-viewresolver-resolver">Resolving views with the
ViewResolver interface</link>" for more details on how Spring looks up
and instantiates a view.) In this example, the
<literal>content</literal> bean is a class that inherits from
<classname>AbstractAtomFeedView</classname>, which returns an Atom RSS
feed. For more information on creating an Atom Feed representation, see
the section Atom Views.<!--Need a correct link or x-ref re the preceding sentence.I couldn't find an "Atom Views" section.--></para>
<para>In the above configuration, if a request is made with an
<literal>.html</literal> extension, the view resolver looks for a view
that matches the <literal>text/html</literal> media type. The
<classname>InternalResourceViewResolver</classname> provides the
matching view for <literal>text/html</literal>. If the request is made
with the file extension <literal>.atom</literal>, the view resolver
looks for a view that matches the
<literal>application/atom+xml</literal> media type. This view is
provided by the <classname>BeanNameViewResolver</classname> that maps to
the <classname>SampleContentAtomView</classname> if the view name
returned is <classname>content</classname>. If the request is made with
the file extension <literal>.json</literal>, the
<classname>MappingJackson2JsonView</classname>instance from the
<literal>DefaultViews</literal> list will be selected regardless of the
view name. Alternatively, client requests can be made without a file
extension but with the <literal>Accept</literal> header set to the
preferred media-type, and the same resolution of request to views would
occur.<!--Can you reword preceding sentence? I don't follow it.--></para>
<note>
<para>If <classname>ContentNegotiatingViewResolver</classname>'s list
of ViewResolvers is not configured explicitly, it automatically uses
any ViewResolvers defined in the application context.</para>
</note>
<para>The corresponding controller code that returns an Atom RSS feed
for a URI of the form <literal>http://localhost/content.atom</literal>
or <literal>http://localhost/content</literal> with an
<literal>Accept</literal> header of application/atom+xml is shown
below.</para>
<programlisting language="java">@Controller
public class ContentController {
private List&lt;SampleContent&gt; contentList = new ArrayList&lt;SampleContent&gt;();
@RequestMapping(value="/content", method=RequestMethod.GET)
public ModelAndView getContent() {
ModelAndView mav = new ModelAndView();
mav.setViewName("content");
mav.addObject("sampleContentList", contentList);
return mav;
}
}</programlisting>
</section>
</section>
<section xml:id="mvc-flash-attributes">
<title>Using flash attributes</title>
<para>Flash attributes provide a way for one request to store attributes
intended for use in another. This is most commonly needed when redirecting
— for example, the <emphasis>Post/Redirect/Get</emphasis> pattern. Flash
attributes are saved temporarily before the redirect (typically in the
session) to be made available to the request after the redirect and
removed immediately.</para>
<para>Spring MVC has two main abstractions in support of flash attributes.
<classname>FlashMap</classname> is used to hold flash attributes while
<interfacename>FlashMapManager</interfacename> is used to store, retrieve,
and manage <classname>FlashMap</classname> instances.</para>
<para>Flash attribute support is always "on" and does not need to enabled
explicitly although if not used, it never causes HTTP session creation. On
each request there is an "input" <classname>FlashMap</classname> with
attributes passed from a previous request (if any) and an "output"
<classname>FlashMap</classname> with attributes to save for a subsequent
request. Both <classname>FlashMap</classname> instances are accessible
from anywhere in Spring MVC through static methods in
<classname>RequestContextUtils</classname>.</para>
<para>Annotated controllers typically do not need to work with
<classname>FlashMap</classname> directly. Instead an
<interfacename>@RequestMapping</interfacename> method can accept an
argument of type <interfacename>RedirectAttributes</interfacename> and use
it to add flash attributes for a redirect scenario. Flash attributes added
via <interfacename>RedirectAttributes</interfacename> are automatically
propagated to the "output" FlashMap. Similarly after the redirect
attributes from the "input" <classname>FlashMap</classname> are
automatically added to the <interfacename>Model</interfacename> of the
controller serving the target URL.</para>
<sidebar xml:id="mvc-flash-attributes-concurrency">
<title>Matching requests to flash attributes</title>
<para>The concept of flash attributes exists in many other Web
frameworks and has proven to be exposed sometimes to concurrency issues.
This is because by definition flash attributes are to be stored until
the next request. However the very "next" request may not be the
intended recipient but another asynchronous request (e.g. polling or
resource requests) in which case the flash attributes are removed too
early.</para>
<para>To reduce the possibility of such issues,
<classname>RedirectView</classname> automatically "stamps"
<classname>FlashMap</classname> instances with the path and query
parameters of the target redirect URL. In turn the default
<classname>FlashMapManager</classname> matches that information to
incoming requests when looking up the "input"
<classname>FlashMap</classname>.</para>
<para>This does not eliminate the possibility of a concurrency issue
entirely but nevertheless reduces it greatly with information that is
already available in the redirect URL. Therefore the use of flash
attributes is recommended mainly for redirect scenarios .</para>
</sidebar>
</section>
<section xml:id="mvc-construct-encode-uri">
<title>Building <literal>URI</literal>s</title>
<para>Spring MVC provides a mechanism for building and encoding a URI
using <classname>UriComponentsBuilder</classname> and
<classname>UriComponents</classname>.
</para>
<para>For example you can expand and encode a URI template string:</para>
<programlisting language="java">UriComponents uriComponents =
UriComponentsBuilder.fromUriString("http://example.com/hotels/{hotel}/bookings/{booking}").build();
URI uri = uriComponents.expand("42", "21").encode().toUri();
</programlisting>
<para>Note that <classname>UriComponents</classname> is immutable and
the <literal>expand()</literal> and <literal>encode()</literal>
operations return new instances if necessary.</para>
<para>You can also expand and encode using individual URI components:</para>
<programlisting language="java">UriComponents uriComponents =
UriComponentsBuilder.newInstance()
.scheme("http").host("example.com").path("/hotels/{hotel}/bookings/{booking}").build()
.expand("42", "21")
.encode();
</programlisting>
<para>In a Servlet environment the
<classname>ServletUriComponentsBuilder</classname> sub-class provides
static factory methods to copy available URL information from a
Servlet requests:
</para>
<programlisting language="java">HttpServletRequest request = ...
// Re-use host, scheme, port, path and query string
// Replace the "accountId" query param
ServletUriComponentsBuilder ucb =
ServletUriComponentsBuilder.fromRequest(request).replaceQueryParam("accountId", "{id}").build()
.expand("123")
.encode();
</programlisting>
<para>Alternatively, you may choose to copy a subset of the available
information up to and including the context path:</para>
<programlisting language="java">// Re-use host, port and context path
// Append "/accounts" to the path
ServletUriComponentsBuilder ucb =
ServletUriComponentsBuilder.fromContextPath(request).path("/accounts").build()
</programlisting>
<para>Or in cases where the <classname>DispatcherServlet</classname> is mapped
by name (e.g. <literal>/main/*</literal>), you can also have the literal part
of the servlet mapping included:</para>
<programlisting language="java">// Re-use host, port, context path
// Append the literal part of the servlet mapping to the path
// Append "/accounts" to the path
ServletUriComponentsBuilder ucb =
ServletUriComponentsBuilder.fromServletMapping(request).path("/accounts").build()
</programlisting>
</section>
<section xml:id="mvc-localeresolver">
<title>Using locales</title>
<para>Most parts of Spring's architecture support internationalization,
just as the Spring web MVC framework does.
<classname>DispatcherServlet</classname> enables you to automatically
resolve messages using the client's locale. This is done with
<interfacename>LocaleResolver</interfacename> objects.</para>
<para>When a request comes in, the
<classname>DispatcherServlet</classname> looks for a locale resolver, and
if it finds one it tries to use it to set the locale. Using the
<literal>RequestContext.getLocale()</literal> method, you can always
retrieve the locale that was resolved by the locale resolver.</para>
<para>In addition to automatic locale resolution, you can also attach an
interceptor to the handler mapping (see <xref
linkend="mvc-handlermapping-interceptor" /> for more information on
handler mapping interceptors) to change the locale under specific
circumstances, for example, based on a parameter in the request.</para>
<para>Locale resolvers and interceptors are defined in the
<literal>org.springframework.web.servlet.i18n</literal> package and are
configured in your application context in the normal way. Here is a
selection of the locale resolvers included in Spring.</para>
<section xml:id="mvc-localeresolver-acceptheader">
<title><classname>AcceptHeaderLocaleResolver</classname></title>
<para>This locale resolver inspects the
<